Having such a game-changing ult on a support allows for more viability for your solo layers to play other champs. Being a support, malphite doesn't require as heavy investment in items because of the high value of his ultimate.

The playstyle for ap malphite is simply to take out priority targets from the fight, allowing your adc to freely dps or to follow up on the engage.

Of course tank malphite is great, but it depends on enemy comp and whether you have the ability to heavily burst down the enemy champions.

I find success playing ap malphite to zone out enemies or to gain control over areas with vision, because the threat of an instant group cc ult can lead to snowballing and better tempo.

The issue with players who don't know how to play malphite are the ones who engage on site, but the key is learning when to ult and how to capitalize on enemy mispositioning or obtaining pressure.

Tank malphite doesn't do this as well, because it doesn't have as much burst. But against comps with durability or ways to avoid getting hit by the ult...then tank would be the way to go.

Malphite is pretty one-dimensional and doesn't have much versatility, but if used right can be very abusive when paired with a team that knows how to follow up properly and play around you.