I mostly use it to say "oh good, there will probably (with a certain % likelihood) be x,000 people who will buy the game within the first week.

I know publishers like Playway do exactly what you describe with all of their <insert niche job here> Simulator series of games, but it's not my thing. I use my wishlist counts to check I'm on the right track, to keep encouraged when development feels like a grind, to help justify putting a certain amount of $$ towards its development. If I was really struggling to get wishlists I would seriously consider changing the game's presentation substantially, or changing to work to something else, but I've not yet been in that situation. But in that case it would be a pretty decent signal that something about how the game looks isn't chiming with potential customers.

There are other useful metrics too, how many YouTube views did the announcement trailer get? Did it get posted to reddit and do okay there? How many people have joined the Discord? And so on and on, so it's not all one thing.

Given the not-perfect-but-pretty-good correlation between number-of-wishlists to number-of-first-year-sales it's a helpful number to help budget/forecast my business.