No, I still don't get it.
Why not just upload an avatar like any other forum ?
Advantage for STW: less load on the hamsters because everyone's pics are held on the gravatar servers.
(Theoretical) Advantage for you: upload one image and it will appear on every Gravatar-enabled website without having to faff about uploading it to each one separately.
but you do to get your own gravatar, as the pic id is keyed to your email address
Yes it is keyed to your email address, but as you point out they use an MD5 hash. In other words they generate a unique number from your email address and send that to Gravatar when requesting your pic.
They don't share your actual email address to gravatar, so there is no breach of data protection.