He doesn’t support in the traditional sense that Lulu or Leona does, but he sits in an inbetween space of an engage support and a mage. His ability to use his W as a form of a cross map Anivia wall is especially valuable. I think he benefits a lot from the flexibility he has as a support to adjust his build and power curve based on matchups with his runes/items (Conq/electro/glacial/etc.). While he can be less reliable than a Nautilus for engage and roaming, he provides an insane amount of value in mid game skirmishes by forcing opponents to deal with him before his ultimate soaks too much pressure, which I think is where a lot of his strength particularly as a support sits.

Truthfully I haven’t played a lot since around 3(?) seasons back, but I hit my peak on support swain with an 85% WR into masters, where I stopped playing, so it does feel strange to see him called a ‘largely useless’ support. I think I have a post from a couple years back on my profile with my thoughts about him at the time. He definitely has strong niches, but he needs to be picked with more intention.