>>23320962Because Weather Ball does hella damage. As a test, I had Castform use Weather Ball(in Hail) and Articuno use Ice Beam on a shitty Tiny Woods monster. Castform is level 50 and Articuno is level 53. Here's the damage:
>Castform: 681>Articuno: 451Enough damage to beat out a Size 4 Legendary. Not only that, but Weather Ball has twice the PP of Ice Beam. It's true that Castform doesn't have much else going for it, but hot damn is it good at what it does.
I conducted my test in Rescue Team, where it's only size 1, but the stats are probably the same in Explorers.