Strong pieces have special abilities like 'Only War', which can buff the defence of nearby pieces.
Proper Chess moves serve as instakill attacks, so if you move your queen (a Space Marine librarian in this case) to take a piece diagonally, your librarian will walk up to an Ork and zap them into chunks immediately. I enjoy directing them to snap fire at the enemy line and open up holes in the enemy defense. Regicide has all the familiar pieces, but now all your pawns can shoot.
Instead it's Regicide mode that has provided amusement between bouts of reading articles on efficient early game development. You can play classic Chess with Space Marines if you like, but you're more likely to find high level AI in dedicated chess games. The Ork is sad because he has been checkmated.
