Check at the beginning of the game page, it says - Release date: October 4th, 2018! (soon!)
When the game was still mostly in beta, it let you play the routes as they were being released and test them out. Currently, there is no "advantage" to it, necessarily, except supporting the creator, since they're an indie developer and are the ones doing the brunt of the work. The only thing I could maybe think of is, they might release voices in batches(since it's being first released without the voices), but we'll have to see honestly.
I had personally purchased it when they had the $51.99 tier, because it came with an artbook and I wanted to give them an amount I thought was fair for all of their hard work. It's up to you which tier you want to pay for, there's no shame in choosing the cheapest one, if you want, but if you wanted to give them more beyond that, that's fine too.