You'll need to get it from the GitHub and side-load it onto your Start9, as it isn't available in the services marketplace yet.

Alby has a guide for this and I'll look for the link for you. What the guide does not mention is that you will need to have a GitHub account to download the package.

Alby Hub on Start9 requires you to be running LND, so it is not a standalone lightning node in this case. It will use your existing LND node's channels for liquidity.

As for why: Setting up and connecting wallets with separate (Alby Hub uses the term "isolated") balances is trivial, and so long as each wallet is connected to a separate Alby account, they will automatically have a lightning address.