It could serve as a reference implementation and as a testbed for finding the optimal UX for deposit and withdrawal, so that exchanges don’t have to design everything from scratch. Additionally, wallet developers can use it for optimally integrating with this UX.
Currently, deposit and withdrawal on TO are working, but I find the implementation very basic in the way that it requires a number of manual steps. Copy the message, open the wallet, paste the message, sign it, copy it, open exchange again, paste the signed message, submit. I am counting 8 steps for the user, if you only count the steps that require moving fingers.
From a user’s standpoint, it would be easier if the slatepack message could be passed to the wallet with a single click, like clicking a grin:// link on the exchange, so that the walllet would be triggered automatically. The next step for the user after the first click would be to confirm signing the transaction in the wallet.
Ideally, the user would be done at this point after two clicks, which means that the wallet would have to pass the signed message back to the exchange automatically.
Maybe this behavior could be achieved with a browser extension that listens for grin:// links that contain the slatepack message and an optional response API address that the wallet can send the signed message to. Not only for exchanges, but also for chats with web interfaces, PMs and so on.
However, a testnet exchange could help with streamlining such UX processes across multiple wallets, so I think it would be better to have than not.