Some franchises have significant fandom overlap and crossover, so somebody might want to use their Marvel wiki account to also do some work on the DC wiki.

It would also make it easier to cross-link incidental mentions. For example, quite a few actors appeared on both Star Trek and Stargate. If the Stargate wiki can write "John DeLancey also played [[Q]]@startrek.wiki", it makes for a less seamful reading experience.