Using mobile phone numbers as a makeshift captcha is the #1 tool any security team has to prevent fraudulent signups. Because they're expensive to get, it puts

Using mobile phone numbers as a makeshift captcha is the #1 tool any security team has to prevent fraudulent signups. Because they're expensive to get, it puts any attack at a baseline cost $x, so many would-be attackers that only stand to gain $y just don't carry out the attack when $y < $x.