They use it as a primary key for linking all your other personal information, use it to geolocate you, use it to sell to marketing lists as a verified number, and use it to cross reference with other sources that leak your phone number.

It’s the “one ring”.

if i had a company and had to collect phone numbers then id just message every one of those phone numbers: BRICK

One thing I would suspect is they leverage third parties and share your phone number to get back additional known data about you or your interests or other activities which other companies have shared. I think in a way it ends up being a connection point for your data across many places.

One of the more legitimate uses is to have a decent way of identifying humans. Most humans only have a very limited amount of phone numbers (usually exactly one) and even extreme cases can only acquire a rather limited amount of them.

Contrast that to most other identification methods such as email or online accounts where a singular entity can create limitless amounts of them, that’s quite a lot better.

Obviously most companies also abuse your phone number for malicious purposes such as tracking, profiling, spam etc.