The only reason I can think of for anyone to change from Gnome to KDE would be.... "you like KDE better."
And the same goes for changing from KDE to Gnome.
Or to see what the other side is like. I was very KDE-centric and didn't think I'd like GNOME at all because of things I'd read. But I knew a lot of people liked it so I wanted to get a feel for it and find out why. Now I like using GNOME very much, as well as other desktop environments and window managers! One of the things I like most about Linux is that I can log into different environments on different days.