I'm guessing here: there is probably a mechanical lock (on the L/G handle) which activates once the Weight-on-wheels switch is activated, this ensures that the landing gear can't retracted on ground. (this is most preferable for obvious reasons :p)
So, in the case of this procedure-point: Most likely, the pilot is to check that the landing gear handle is completely down, and then (not sure on this one) try to put the handle on up-position: the handle should be locked. Thus, you have ensured that the Landing gear handle mechanical stop is fully engaged.
Could be wrong on this
Addition:
On some aircrafts there is a “trigger” around the index-finger-position on the L/G handle. The L/G handle can’t be moved if the trigger isn’t pulled, and held.
I’m not sure if the F/A-18 has this, but it might be that the “mechanical stop” ensures that the trigger can’t be pulled.
This makes it so that the pilot doesn’t have to try to put the L/G handle in the up-position, he (or she) only have to ensure that the trigger is locked: the L/G handle is locked.
So, in DCS-terms: once you have landed, all you have to do is to press "G"