I'm sure it varies from person to person, but as far as I can tell it's some combination of FOMO, peer pressure, and virtual dick measuring

People mostly pre-order for the add ons they put in. It's not uncommon for pre-orders to get beta access, or early access. Sometimes they get even greedier and tie that into the next tier up so you'll pay $80-100 for the game. People don't like to wait/get hit with fomo when they see everyone playing the game already.

It's installed when I want to play it. Usually the counter I hear is that internet is fast enough where that isn't a problem. This is an argument, that gamers use to contend when the idea of online only was considered by Microsoft. The fact that I can preinstall 100GB of data beforehand at my leisure is worth it.

Especially now considering that steam offers free refunds up to 2 hours of gametime or 15 days after release? Realistically on PC there is very little reason to argue against it at this point in my opinion (obviously this is referring to PC, no need to do that for Consoles that don't provide these opportunities.

This isn't really a pro preorder, I just lost my frustrations with it when the risk was mitigated