It helps prevent against partitioning. The IP's given out by the DNS seeds are entirely random, while the IP's discovered by other methods are not so random.

Prior to the DNS seeds existing, Bitcoin used to join an IRC server and get other node IP's from that. Satoshi seemingly stole this code from gnutella, a filesharing protocol. The IRC server admin thought Bitcoin was a filesharing program or a botnet, and banned it from the server. Bitcoin used another IRC server for a while, until DNS seeds were implemented. Clearly the DNS seeds are a much better way than using a centralized chat server. Because it uses DNS, even if the seeds go offline, most nodes can still bootstrap for a period of time until DNS servers clear their caches.

I'm not sure what x1, x5,x9 and xd is.