AndreyKladov: The problem statement is completely misleading. Even thou the marked stone is useful, it is not directly *used* in the further combination. Thus, the problem statement can not be understood directly by someone without the dan skills. I suggest to rewrite it.
Jslate: for newer players, the reason G2 is marked is because it prevents J2 from being atari'd so one is then able to extend at M3 and capture stones. if G2 is not there, the opponent can force you to make one big blob of your stones, and then ur opponent can connect their stones on 1st and 2nd line which can then connect to their 3x stones on the left.
you can play around with on the Review if you want to see it.
It's correct. The marked stone is useful