This design has the really annoying situation that applications closing down can't leave stuff in the clipboard. This results in a quite a frustrating behavior for users and also makes command line tools tricky. wl-clipboard lets you pipe text into the command to copy. The way it works is that it daemonizes in the background to fulfill the copy request. It either cancels on the first paste (command) or until some other application copies something into the clipboard. It's not completely awful design, but I'm surprised that the protocol does not offer at least the option to move content into the clipboard and exit.