You can create front facing software that only interfaces with blockchain(s). To create a wallet you create a pub + private key pair. You can then sign tx's with that. A DApp can enable transfer of assets between addresses, calling smart contract functions in deployed smart contracts, and the frontend keeps you updated on the tx statuses. Essentially they are created for privacy and security reasons, and to execute scripting code from a mobile app, which is stored at a deployed smart contract address. I'm not sure what Apple's motivation is here, compliance or legal issues would probably be their biggest complaint? Or is this their attempt to squash an app store within their App Store?