It keeps the other projects hosted on the same server or network online. Blackhole routes are pushed upstream to the really big networks and they push them to their edge routers, so traffic to the affected IPs is dropped near the sender's ISP and doesn't cause network congestion.

DDoSers who really want to cause damage now target random IPs in the same network as their actual target. That way, it can't be blackholed without blackholing the entire hosting provider.