The Behemoth is an experimental mobile heavy canon equipped a powerful disintegrator gun and three secondary beam lasers. While its main cannon lacks the instant-kill power of a Commander's D-Gun ability, it's still able to kill a heavy tank in one blast, and quickly obliterate even the heaviest T3 units.
Use it to spearhead endgame assaults against heavy fortifications.
Behemoths can tank through multiple direct hits from the heaviest weapons and remain combat-effective. They are still vulnerable to being outranged, and have no weapon vs. air units. Keep them well supported with other forces to cover their flanks and weaknesses.
In many cases, they are a better option than Juggernauts, although they lose out in mobility and all-around capability.