Another thing to consider is that Water-types tend to have bigger movepools than Grass-types, and the latter tends to have quite limited options unless if it's something with a dual-type. Water-types have fairly useful moves and some do learn some interesting stuff here and there.
Anyways, I have never used a Manaphy myself but I know times I used one on Showdown. It's one of only two Pokemon to learn Tail Glow naturally, and once it gets a boost off it can be quite threatening if you don't take it out fast enough. It also learns a lot of fairly good special moves which takes a huge advantage of it.
