In what situation would I want to have multiple creatures block one attacker? If one creature can absorb all of the damage, why subject more creatures to the excess damage? Other than trample, is there any advantage?
The simplest case is using multiple weaker creatures (say, two 1/1s) to block and kill a single 2/2. Using only one creature would (under normal conditions) be rather bad.
Even better, use two 1/3s to block and kill a single 2/2, while your creatures survive combat unharmed (barring any instants being cast).
In addition to Glorfindel's answer of gang-blocking with weaker creature to kill a stronger attacker, there are some edge-cases where it is beneficial to you to have your creatures deal damage.
An example for a case for this happens semi-regularly in the current standard Tokens deck where one player has a lot of 1/1 Lifelink tokens. Say player A has 10 1/1 Lifelink soldiers and they are attacked by a 1/1 goblin token (say from Goblin Warboss).
- If Player A blocks the 1/1 goblin with a single lifelinker he will lose 1 solider and gain 1 life.
- If Player A blocks the 1/1 goblin with all his lifelinkers he will lose 1 solider and gain 10 life.
Another reason to block with more creatures than you expect to need is for insurance against combat tricks (instants played after declaring blockers but before combat damage is dealt). If I'm at very low life, I will often over-block to prevent trample-granting buff instants from killing me. Over-blocking can also help to ensure that you kill an attacker - if someone attacks with a 1/1 when I have two 3/3s on the battlefield, I might block with both, as it's likely they have some instant like a +3/+3 which would hurt if I block with just one of my creatures, but might be a fair tradeoff if I block with both. Sure, it looks like the one blocker will be enough when declaring blockers, but it's very useful to consider what might happen before damage is dealt.
A good question, and the answer is, yes. Not always but there are several cases where blocking with multiple creatures benefits you over blocking with just one. Malco and Glorfindel have given reasons which I will mention here too for completeness.
As you said trample is a big reason to block with multiple creatures, you need to put enough toughness in front of the creature to prevent the rest getting through.
As Glorfindel said, killing the attacker is another reason, weenie decks have alot of small creatures, often tokens, that are expendable, throwing a few in the way of a big threat may lose you more creatures, but the creature they lost may matter much more.
As Malco said, lifelink effects work when you overkill, unlike some other combat card games, you deal the full power of your creatures to whatever you block, not just enough to kill that creature. The overkill when you have lifelink gains you more life.
Playing instants in MTG during combat is known as combat tricks, if a person casts Giant Growth or something similar on their attacker during combat, your blocker that was bigger than the attacker may now be the smaller creature. If you block with a second creature or third, you might still lose some creatures in the exchange, but the extra damage may be enough to still kill their creature, or they may not spend the spell on a losing combat.
You may have effects that care about when you deal damage to creatures. One example of this I can think of is Greatbow Doyen, which makes your archers deal damage to the creature's controller equal to the damage they do to creatures, you can block with a lot of archers to really punish the player for attacking, if you don't win outright from all that non-combat damage.
There are creatures that have effects that happen specifically when you block with that creature, such as Megalonoth, Engulfing Slagwurm, and Goldenglow Moth. Including these creatures in a block, even when it isn't needed to kill the creature will give you the benefit of their effect.
Menace is an ability on creatures that force you to block with 2 or more creatures. Even with a creature as weak as Goblin Glory Chaser though you only need one damage to kill their creature, this forces you to block with at least two or not at all.
You may simply have to block with everything you have. A creature like Prized Unicorn or something enchanted with Lure forces you to block a single of your opponent's creatures with everything. This is usually done to let the rest of their army through to hit directly.