Tangrowth's main purpose in competitive battling is primarily as a pivot, and a defensive pivot at that.
This means that it uses its Regenerator ability to switch in and out as and when needed with fewer consequences than usual, as it regains a good chunk (~33%) of its HP every time you switch it out.
Unfortunately, it is not good enough to stay in and sweep, owing to poor speed and typing.
It's best suited for sponging hits from common threats such as Ash-Greninja, Mega Diancie, Garchomp, Tapu Koko and several others due to typing, bulk, and ability. In doing so, it forces switch outs, making your opponent lose momentum, so you can stop their onslaught by switching out to your counters to their possible switch-ins (Fire types, Flying types, Mega Venausaur are the most common), so your own sweepers/wallbreakers can switch in safely.
As for using Knock Off, it is more effective in ensuring opposing defensive Pokemon such as Ferrothorn / Chansey are crippled by getting rid of Leftovers, Rocky Helmet, or Eviolite. It is not as effective in stopping wallbreakers as they tend to carry Mega stones or Z crystals, although getting rid of a Choice item might benefit your team in the long run.
Keeping all this in mind, you're best off using it mid-game to disturb the opponent's flow, as it is underwhelming as a lead, and a liability in the late game, meaning ideally it should never be the first or last Pokemon to be sent out. That being said, that HP Ice, and Giga Drain/Power Whip and maybe even Earthquake can be used to finish off weakened foes.
Try using it a few times to check if it fits your style of playing. If you don't mind switching a lot to maintain advantage / momentum, this could be the defensive pivot you're looking for. If instead you prefer stall/set-up sweeping or wallbreaking without trying to play too many mind games, this Pokemon might not be for you. Many players find it a hit or miss, being the crucial Pokemon in some games, and completely useless in others, so understanding the nature of the meta you're playing in makes all the difference.
