To OP: it does add significant block to spammers so they may only create 5,000 accounts slowly instead of millions of accounts quickly.
Most email services require a phone number or other email service to sign up if there is anything fishy about your connection/browser, and there can be hundreds of things, even just some specific browser extension installed, or you using the same mobile network IP as a spammer.
And there are services where you can buy a real temporary 4G/5G number (non-VoIP) for a service for less than a dollar per use. Once a number has been used for that service it still has every other X number (thousands) of services it can be used for before it's retired from the pool. But that does add up after a while if you're making tens or hundreds of thousands of accounts.
Requiring email of phone numbers isn't always something virtuous like stopping spam. It's also often about first party data acquisition. Even if some people use burner accounts, most won't, and that means more data to share with advertisers.
I used to get a bunch a couple years ago. Since then, I've deleted all Instagram, Facebook, Reddit, Snapchat, etc. I only use Lemmy. No more spam or phone calls.
I also use separate phone and phone number for banking and actual important stuff so that phone is off until I need it so ymmv
For Americans, yes... but everywhere else in the world phone numbers are cheap as dirt. When I was overseas in Spain my local number cost me 3 euros/month