Why some big streaming websites use subdomains such as this?

Hey Friends,

I have one question that has been puzzling me recently,

I've seen big streaming websites such as 9anime and animeflv.net basically change their domains like this:

For example,

www3.9anime.to, then later to > www4.9anime.to, www5.9anime.to and so on

the same goes for animeflv.net, www3.animeflv.net, to www4.animeflv.net and so on,

Why do they do that, what's the purpose of it?

I'd appreciate your answer!