To curb abuse.

Number spoofing is trivial, and SIM cards can be bought retail for as little as 1€ in the EU and many countries around the world.

You can buy them in bulk on shady sites for as little as 5c per pop.

Most abuse happens on an industrial scale, and it's trivially easy and practically free to bypass this kind of "security" feature.

Right, but 1€ isn't free. So if you manage to spam 1M people and make less than $0.05 * 1M = $50,000 then you are losing money.

Thus no spam on signal.

And to curb privacy / anonimity.

Hardly an actual mitigation. Temp phone numbers are in abundance and a dedicated spammer can definitely overcome it

No system is perfect. It’s about making it time-consuming and financially expensive to the spammer.

What types of abuse it really curbs?

Those that depend on creating new accounts to replace blocked ones.

Getting a new phone number isn't expensive, but it's infinitely more expensive than zero. And if a service is willing to block the phone companies that offer the cheapest new numbers, the price rises again.