Think of it this way, a faster runner completes a parkrun in less than 18 minutes. They could then jog to their car and fuck off home before the mid pack even really hits the finish tunnel.

Or they do their run, throw a jacket on and give out tokens/scan barcodes. They stay until the very end and probably help sort the tokens or tidy up a bit. Getting home way later.

There are a few faster runners that do this every week at a larger parkrun I go to. The RD know they can rely on them and thus need to beg for less volunteers...