Surge gives you an alternate casting cost for a card if you or a teammate has cast another spell this turn. Paying the Surge cost also gives you an added benefit. Crush of Tentacles on its own is little more than an Devastation Tide type effect - making everyone scoop up all of their nonland permanents. But when it’s cast with its Surge cost for 3UU, it not only does that, but also puts an 8/8 Octopus token into play for you. That's a pretty solid card right there.
As a Mythic Rare, Crush of Tentacles doesn't blow anyone away, but it's certainly a playable card. It was playable in some competitive Control decks during its time in Standard. Also, it should be a very good card in Commander, and in casual play, for years to come.