@markdudov It allows a remote client/peer that has connected into the wireguard "network" to also use that network's internet connection.
This could be for a variety of reasons. Most common is that if the WG tunnel is configured to force all traffic through it then without this rule, you will not be able to access anything on the public internet. The remote peer will then appear to be on that same WG network even though it is connecting into it from somewhere entirely different.