It's an additional cost
Hugh McMullen, former Wizards Director of Communications, told me in 2016 that it is important to the game to offer a representation of anyone who might play it
When Hastorr is sent from the monster zone to the graveyard, it can equip itself to an opponents monster, preventing that monster from attacking and negating its effects for as long as Hastorr remains equipped
Its still an insanely difficult-to-pull chase card featuring a memorable Pokmon which lends it plenty of value
When attacking, this card can Trash one of the opponent's 2-cost Characters and since we have a lot of cost-reduction cards, we can put higher cost targets in range for OP07-091-monkeydluffy to Trash
Igglybuff's fairy typing, added in Generation VI with Pokmon X and Y, further enhances its appeal by providing it with a unique type combination that offers interesting strategic options in battles