There's good feedback in the thread already. Going to add to it.
He has one of the fastest jungle clears in the game with cinderhulk+ W max.
He is an amazing splitpusher since he can clear waves of minions so quickly with his W + sunfire cape. Hard for the enemy to do anything about him since he's super tanky, has a low cooldown ult which heals him for a ton while making him run fast, has a spammable slow on his Q.
He is unkillable lategame. Very good for facetanking tower aggro (Plus the enemy team) while your carries and assassins dive. He's very useful in poke comps, since he can poke/peel with his Q, but also take (and heal off) tower aggro if you don't want to wait for the next wave to poke down a tower.
He is super tanky, so good for tanking dumb objectives (Baron/Dragon) that would be doing a lot of damage to your teammates. Think two-manning baron with your kalista ADC at 20 mins tanky.
He is good for peeling tanks. Turn on your W, throw cleavers at the tankline trying to get past your frontline to hold them in place. Your cleaver does %hp damage, which does a lot to that 5k HP volibear trying to run at your adc.
So he's useful, but not in every team comp. I wouldn't blindly pick him every game and expect to win since there are very specific strategies that his kit works well with.
My $0.02.