Reduce attack surface/abuse. There are plenty of services that maintains lists of “bad IPs” or “VPN IPs”. You can subscribe to them and get the list and block it. Most bots, spammers, attackers etc always run behind some type of VPN or Tor depending on what you’re doing and how much you’re trying to hide your trail.
Most services don’t go out of their way to block VPN IPs because it’s just a support headache/nightmare. Some services like Netflix are legally obligated to block well know VPN IPs