Got a spare set of alloys for using off-road and have just put a set of 235/85 R16 Recip Trails on them

. The alloys didn't come with the centre caps and the caps from my 15" alloys don't clip in properly. The rim depth is deeper on the 16" alloys and the caps would probably pop of at speed or off-road.
Do I need these? Are they purely cosmetic or are they designed to keep the shit out of the centre of the wheel?
Can anyone enlighten me please
Steve