Guild Wars 2

To decrease load on the servers and increase speed of patching for the users? It can be situational, such as when the patch is small, it won’t use P2P at all. Even Microsoft is planning to use peer-to-peer technology to send updates to users.

http://www.theverge.com/2015/3/15/8218215/microsoft-windows-10-updates-p2p

Except it’s a problem for players with metered accounts. Microsoft tries to identify these and not use them for P2P updates but also allows users to manually shut it off.

But the primary purpose is to distribute an update on internal networks using P2P.

As you said, you could turn it off if you don’t want it. League of Legends has been doing this, Battle.net is also doing it. Ironically, ArenaNet came from Battle.net. One step forward, then dun goofed and stumbled five steps back.