They can fundamentally break most asymmetric encryption, which is a good thing iff you want to do things that require forging signatures. Things like jailbreaks Apple can't patch, decryption tools that can break all E2E encryption, being able to easily steal your neighbor's Facebook login at the coffee shop...

Come to think of it, maybe we shouldn't invent quantum computers[0].

[0] Yes, even with the upside of permanently jailbreakable iPhones.