Collaborations in music are a special time for bands and fans.
For the artists involved, it gives them a chance to work with friends, heroes, inspirations, or even increase their stock, working with bigger names to get their music heard by a greater audience. For fans, they get to see their favourite artists experiment a bit and potentially work with other artists they like (lawd praise that Ihsahn/Matt Heafy collab).
Sometimes they work, and they are lucrative for all involved, and everybody's happy. And rich. And the world is a better place for it.
Other times we get Lulu. And then world just gets sad...
Typically, collaborating artists stay within their own worlds, i.e. Bring Me the Horizon working with Architects bro Sam Carter, or Cheap Trick's Robin Zander guesting on fellow lady-lovin' glam metal stars Steel Panther for their cover of "She's Tight", but occasionally artists will work outside their comfort zone to varying degrees of success.
I would give examples, but then that would take away the purpose of this list...