One point that people without app experience probably aren't considering - on iOS, having a large number of downloads (particularly in the first week after launch) can really boost your keyword rankings and ASO. Without this boost, climbing up the keyword rankings can be extremely difficult.

Apple's pre-order feature does precisely what I'd expect - automatically downloading the app when it's available, which helps jumpstart the flywheel explained above.

I would have expected Google's pre-registration feature to work similarly. Thanks to OP for spending the $$ to find out this isn't the case!