Most cards can only attack or use their abilities once per turn, and tapping the card is how you show that this once-per-turn power has been used up
Heck, in the right metagames, you could totally run this bad boy maindeck, disrupting: Emrakul, the Aeons Torn in a Nahiri, the Harbinger deck Lightning Storm in an Ad Nauseam / Angels Grace deck Valakut, the Molten Pinnacle in a Scapeshift deck with even just a little time Its probably not a major change to the format or anything, but I generally like that it promotes one of the fringe strategies, giving them another tool for fighting against unfair decks
Take the following situation, for example, even if it may be a little hard to pull off: You go second, starting with Thundurus-EX against your opponent's Pokemon-EX
The issue: Most cards have printing inconsistencies or uneven frame widths