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.