As a general rule, experiences magic players wait until the last second something will be useful before using their mana and abilities. The more mana you have, and the more spells in hand you have, the less your opponent knows about what you can do, and that's always an advantage. The last second to cast anything sorcery speed is the second main phase, the last second to cast something instant speed is the end of your opponent's turn before yours begins.
Players don't use Heliod, Sun Crowned to give lifelink to a creature until right before combat damage, there's no point to having lifelink until damage is going to happen. Players don't sacrifice Fanatical Firebrand as soon as they play it, they wait until they are using it to block, or it is about to die for some other reason to get a little extra out of it.
Lurrus is used this way too. Players will wait to cast Lurrus until the last second, when they want to play a card in their graveyard or they are otherwise ready to end their second main phase. If you're going to be casting something like Dead Weight using Lurrus, casting Lurrus before combat then immediately using Dead Weight to weaken or kill an opponent's creature matters. If you're going to be playing a creature with Lurrus, unless that creature is going to have haste or some other ability that will change the coming combat (like casting Charmed Stray out of the graveyard to pump ones already on the battlefield), you usually wait until you're ready to end your turn - this isn't about speed, it's about priority and strategy, and the player who's turn it is always gets priority before their opponent does.