Which meant that when Wizards wanted to make instants, sorceries, enchantments, etc. with creature subtypes, they couldn't. At least, not easily. Tribal was a work around. By adding the Tribal type, it allowed Wizards to do an end run around their rules without having to rewrite their rules (I can only imagine what weird edge cases would crop up if any card type were allowed to have creature subtypes).
For reference, there are only certain subtypes a card with a certain type is allowed to have.
205.3a A card can have one or more subtypes printed on its type line.
205.3b Subtypes of each card type except plane are always single words and are listed after a long dash. Each word after the dash is a
separate subtype; such objects may have multiple types. Subtypes of
planes are also listed after a long dash, but may be multiple words;
all words after the dash are, collectively, a single subtype. Example:
“Basic Land — Mountain” means the card is a land with the subtype
Mountain. “Creature — Goblin Wizard” means the card is a creature with
the subtypes Goblin and Wizard. “Artifact — Equipment” means the card
is an artifact with the subtype Equipment.
205.3c If a card with multiple card types has one or more subtypes, each subtype is correlated to its appropriate card type. Example:
Dryad Arbor’s type line says “Land Creature — Forest Dryad.” Forest is
a land type, and Dryad is a creature type.
205.3d An object can’t gain a subtype that doesn’t correspond to one of that object’s types.
205.3e If an effect instructs a player to choose a subtype, that player must choose one, and only one, existing subtype, and the
subtype must be for the appropriate card type. For example, the player
can’t choose a land type if an instruction requires choosing a
creature type. Example: When choosing a creature type, “Merfolk” or
“Wizard” is acceptable, but “Merfolk Wizard” is not. Words like
“artifact,” “opponent,” “Swamp,” or “truck” can’t be chosen because
they aren’t creature types.
205.3f Many cards were printed with subtypes that are now obsolete. Many cards have retroactively received subtypes. Use the Oracle card
reference to determine what a card’s subtypes are. (See rule 108.1.)
205.3g Artifacts have their own unique set of subtypes; these subtypes are called artifact types. The artifact types are Clue, Contraption,
Equipment (see rule 301.5), Fortification (see rule 301.6), Treasure,
and Vehicle (see rule 301.7).
205.3h Enchantments have their own unique set of subtypes; these subtypes are called enchantment types. The enchantment types are Aura
(see rule 303.4), Cartouche, Curse, Saga (see rule 714), and Shrine.
205.3i Lands have their own unique set of subtypes; these subtypes are called land types. The land types are Desert, Forest, Gate, Island,
Lair, Locus, Mine, Mountain, Plains, Power-Plant, Swamp, Tower, and
Urza’s. Of that list, Forest, Island, Mountain, Plains, and Swamp are
the basic land types. See rule 305.6.
205.3j Planeswalkers have their own unique set of subtypes; these subtypes are called planeswalker types. The planeswalker types are
Ajani, Aminatou, Angrath, Arlinn, Ashiok, Bolas, Chandra, Dack,
Daretti, Davriel, Domri, Dovin, Elspeth, Estrid, Freyalise, Garruk,
Gideon, Huatli, Jace, Jaya, Karn, Kasmina, Kaya, Kiora, Koth, Liliana,
Nahiri, Narset, Nissa, Nixilis, Ral, Rowan, Saheeli, Samut, Sarkhan,
Serra, Sorin, Tamiyo, Teferi, Teyo, Tezzeret, Tibalt, Ugin, Venser,
Vivien, Vraska, Will, Windgrace, Wrenn, Xenagos, Yanggu, and Yanling.
205.3k Instants and sorceries share their lists of subtypes; these subtypes are called spell types. The spell types are Arcane and Trap.
205.3m Creatures and tribals share their lists of subtypes; these subtypes are called creature types. The creature types are Advisor,
Aetherborn, Ally, Angel, Antelope, Ape, Archer, Archon, Army,
Artificer, Assassin, Assembly-Worker, Atog, Aurochs, Avatar, Azra,
Badger, Barbarian, Basilisk, Bat, Bear, Beast, Beeble, Berserker,
Bird, Blinkmoth, Boar, Bringer, Brushwagg, Camarid, Camel, Caribou,
Carrier, Cat, Centaur, Cephalid, Chimera, Citizen, Cleric, Cockatrice,
Construct, Coward, Crab, Crocodile, Cyclops, Dauthi, Demon, Deserter,
Devil, Dinosaur, Djinn, Dragon, Drake, Dreadnought, Drone, Druid,
Dryad, Dwarf, Efreet, Egg, Elder, Eldrazi, Elemental, Elephant, Elf,
Elk, Eye, Faerie, Ferret, Fish, Flagbearer, Fox, Frog, Fungus,
Gargoyle, Germ, Giant, Gnome, Goat, Goblin, God, Golem, Gorgon,
Graveborn, Gremlin, Griffin, Hag, Harpy, Hellion, Hippo, Hippogriff,
Homarid, Homunculus, Horror, Horse, Hound, Human, Hydra, Hyena,
Illusion, Imp, Incarnation, Insect, Jackal, Jellyfish, Juggernaut,
Kavu, Kirin, Kithkin, Knight, Kobold, Kor, Kraken, Lamia, Lammasu,
Leech, Leviathan, Lhurgoyf, Licid, Lizard, Manticore, Masticore,
Mercenary, Merfolk, Metathran, Minion, Minotaur, Mole, Monger,
Mongoose, Monk, Monkey, Moonfolk, Mutant, Myr, Mystic, Naga, Nautilus,
Nephilim, Nightmare, Nightstalker, Ninja, Noggle, Nomad, Nymph,
Octopus, Ogre, Ooze, Orb, Orc, Orgg, Ouphe, Ox, Oyster, Pangolin,
Pegasus, Pentavite, Pest, Phelddagrif, Phoenix, Pilot, Pincher,
Pirate, Plant, Praetor, Prism, Processor, Rabbit, Rat, Rebel,
Reflection, Rhino, Rigger, Rogue, Sable, Salamander, Samurai, Sand,
Saproling, Satyr, Scarecrow, Scion, Scorpion, Scout, Sculpture, Serf,
Serpent, Servo, Shade, Shaman, Shapeshifter, Sheep, Siren, Skeleton,
Slith, Sliver, Slug, Snake, Soldier, Soltari, Spawn, Specter,
Spellshaper, Sphinx, Spider, Spike, Spirit, Splinter, Sponge, Squid,
Squirrel, Starfish, Surrakar, Survivor, Tetravite, Thalakos, Thopter,
Thrull, Treefolk, Trilobite, Triskelavite, Troll, Turtle, Unicorn,
Vampire, Vedalken, Viashino, Volver, Wall, Warrior, Weird, Werewolf,
Whale, Wizard, Wolf, Wolverine, Wombat, Worm, Wraith, Wurm, Yeti,
Zombie, and Zubera.
205.3n Planes have their own unique set of subtypes; these subtypes are called planar types. The planar types are Alara, Arkhos, Azgol,
Belenon, Bolas’s Meditation Realm, Dominaria, Equilor, Ergamon,
Fabacin, Innistrad, Iquatana, Ir, Kaldheim, Kamigawa, Karsus,
Kephalai, Kinshala, Kolbahan, Kyneth, Lorwyn, Luvion, Mercadia,
Mirrodin, Moag, Mongseng, Muraganda, New Phyrexia, Phyrexia, Pyrulea,
Rabiah, Rath, Ravnica, Regatha, Segovia, Serra’s Realm, Shadowmoor,
Shandalar, Ulgrotha, Valla, Vryn, Wildfire, Xerex, and Zendikar.
205.3p Phenomenon cards, scheme cards, vanguard cards, and conspiracy cards have no subtypes.
As far as actual mechanics Tribal adds, the answer is it doesn't. It was a work around for a very large set of cards and rules that prevented having to change one of the game's most fundamental rules.