It'll be quite useful to select a subset of all tabs opened in a window and either close them or add them to some group or to define them as homepage(s). think of situations where single-window mode is activated. one cannot open up a separate window to purposely open the desired set of tabs.
allowing a subset of tabs to be selected could also make organizing the tabs into separate windows by using "Detach Layer group" kinda functionality. i haven't seen firefox have this feature yet