The reason would be to check out that ship to see if it's one I would like to take a cruise on.
My thinking was that all the access stuff is computerized so they could easily allow the Dream key cards to scan in on the Wonder and they would know exactly which Dream passengers were aboard and when they got off. I'm sure the
DCL ships use the same system so seems like it would be technologically possible. It's similar to leaving Magic Kingdom and going to Epcot, in a way.