There are some real bad actors behind IP blocks, or hosting providers that have no problem hosting them nor take actions on abuse reports. Referrer spamming, searching for vulnerabilities (some of them with very big URL list to try), misbehaving crawlers, or just plain DoS are some of the ways they may sites, specially the ones serving dynamic content. This space is usually fixed and used by servers, or VPNs exitpoits. Blocking all the blocks associated to their autonomous systems would avoid to put in the rules a lot of /24.<p>But then there are residential IP blocks, specially some with dynamic enough IPs or NATed ISPs. Some people of those blocks may have hostile or clueless behaviour, some may be used as proxy because malware or because they intentionally installed some of the residential proxy servers agents. There you may be blocking legitimate visitors, if a few clients of some ISP are very active you may end blocking a lot of innocent people. And, in this case too, you can target the IP blocks of its autonomous system if you feel that from there you only get bad traffic.<p>But in the end, is your site. you are free to decide to block what you understand that are bad neighbourhoods.