A phone number used to be an address for placing a call. Today it recovers accounts, finds friends and proves who you are. Each service has a reasonable-sounding reason for asking, but together those reasons land on the user as one burden: a single number that links every service's version of you.
Reason 1: to keep out fake accounts
Because a number usually takes a contract or an identity check to obtain, it makes fake accounts hard to mass-produce. For a service it is the simplest spam defence available. The effect weakens as more services ask, though, because every additional service is another place the number can leak from.
Reason 2: to find your friends
Upload your contacts and the service builds "people you may know" from the numbers in them. Convenient for the uploader, but the address book being uploaded contains other people's numbers, and they never agreed. Number-based friend finding is a structure in which everyone's number is shared with everyone else, and opting out depends on whether the service offers a setting to turn off discovery by number.
Reason 3: to recover your account
Confirming by text when a password is forgotten is simple. But once the number is the recovery method, whoever takes the number takes the account. That is why SIM-swap fraud targets financial accounts. When every recovery path converges on one number, that number becomes the most valuable thing to attack.
What the user gives up
Numbers are hard to change. Changing one means re-registering every recovery method and getting every acquaintance to change the number they have saved, so in practice the number becomes a permanent identifier: leaked once, valid everywhere. Reading it out to someone you met an hour ago, or writing it on a name tag, hands that permanent identifier to people you barely know.
How to reduce it
You cannot avoid phone numbers entirely, but you can separate the places that truly need one from the places that ask out of habit. Choose services that sign you up without a number, use an address you can end instead of a number for people you have just met, and do not let every recovery path converge on one number. Every number you do not hand over shrinks the range in which it is valid. The guides on event group chats and bios and name tags show where that line usually falls.