Dossier
The frag grenade clears the problem of multiple targets in a confined space. One throw, correctly placed, handles a room that would require multiple trigger pulls and multiple exposures with any other weapon in the arsenal.
Vehicles are not immune. An exterior detonation is sufficient to disable most unarmoured vehicles. An interior detonation through an open window ends the vehicle and everyone in it.
The four-second fuse is the variable. Too short and the throw range is limited. Too long and the target moves or picks it up. Placed rather than thrown in situations where accuracy matters more than distance. On a timer, four seconds is long enough if the placement is correct.