It offers a lot of value for multiple different reasons, and even presents itself as a decent flier on rate in the scenario where your opponents dont have any activated abilities to take advantage of
Almost always, it's mismatched expectations
Ox of Agonas Remember when I said that some cards want to be discarded
Gandalf the White, for example, is designed to mainly care about legends, but he can also break the famous artifact Isochron Scepter, as noted in this Reddit thread where users recognized the potential that Gandalf's ability adds to the Isochron Scepter
Not only that, but the Creature gains Indestructible until end of turn whenever you cast a spell, making combat a nightmare for your opponent if you have open mana
Thats a lot of hoops to jump through just to get a legend onto the field, but at least it lasts until your next turn, so cards like Jin-Gitaxias, Progress Tyrant have a whole round to punish your opponents before they revert back to whatever they were