volibear has built in attack speed, and his ult buffs each auto attack, and his w does tons of damage despite building tanky and does more damage the more hp they are missing. so it is a great executioner type of move, like cho's garen's darius's ultimates. except volibear's w is not nearly as powerful. but can be used from short skirmish to short skirmish, or near the end of the fight to really bring a hurtful move.
he is not that good though, most popular tops will beat a volibear. volibear can only reliably beat the less popular top choices like nasus, irelia, poppy, renekton. the only moderate top volibear can match up ok against is xin. but people like garen darius elise cho malphite panth olaf shen teemo. they all do well against a volibear. those executioner people just got stronger executes, due to it being on their ultimates. you can win against them, but it will be hard very hard. any one with a brain can totally out play volibear though. even at low elos they will counter your passive with ignite and you know just out play you, and they understand that you will try to execute them if they low, and that you are easy to kite.