Because they might give your phone number to someone else, and then you don't have any more control over who has your personal information. That's how I look at it at least.

I can see someone who is in a very sensitive situation, like trying to escape domestic abuse, or starting a protest, might want the anonymity but still need the communication.

I fully agree about blocking. Giving someone your phone number is not as scary as the internet portrays it to be. In fact, people are hypocritcal, because they will give their number to a business where mutiple real people can access their info, but if they give their number to a person, they delusionally think stalker, harasser. People think way too much of themself. If I have a person's name and phone number, if I don't speak to them, I will get rid of their number. People don't care about getting a random number as much as the internet portrays it.