It isn't necessary.
The pros that I see are: it protects the bottom glass of the tank, if using sand you can get the look of deeper sand without having to actually have deep sand (my reason), if going bare bottom it keeps the light from shining down thru the glass and into the stand - coralline will do this after a while though, you can drill the starboard and glue in rods to be used to elevate the rock so that powerheads can keep the area beneath the rock from collecting waste - I think this works best if you are going bare bottom.
The cons: if going BB the starboard is not as smooth as glass so detritus has a tendency to kind of stick to it (I read this on several threads)
I am sure there are other pros and cons but this is what comes to mind.