Trample is useful in general, but particularly so with monarch, as it lets you easily land combat damage on your foe to reclaim your title if stolen.
Behemoth provides a versatile unit his 5/5 stats alongside trample (which deals pierce damage through blockers) help in battle while his monarch ability lets your lands tap for additional mana while you're king of the hill! His six mana cost may turn some away, but thanks to green's various ramping and free-creature gimmicks like 'Norwood Priestess' and 'Mosswort Bridge', you'll have Behemoth out before you know it.
