Why are there holes in the orbital sandpaper?

The holes in the orbital pads allow the sander to suck away the sawdust and throw it in the dust collection bag or chamber. If this does not occur, then dust gets trapped under the sander disk and keeps it from sanding efficiently. The hole configurations of the discs are standardised to suit the hole configuration on most mainstream orbital sanders ie a 125mm 5″ disc sander will usually have an 8 hole configuration. The presence or absence of any holes on the disk must be compatible with the sanding machine that you are using.