It is a convenience thing. If you need to string together multiple ground connectors from several instruments and from parts of your DUT then the 4mm banana socket on the front makes it easier to connect the oscilloscope, too. You don't have to crawl behind the instrument and look for a ground screw.
However, and that is a big however, this is no substitute for properly connecting the ground on your probe to the DUT, especially not for high-frequency signals. It is also no proper substitute for any safety feature. The oscilloscope itself should be properly connected to PE, independent of the use of that socket. Further, with a bunch of 4mm banana test leads it is easy to set up all sorts of funny ground loops and build undesired antennas.