Another sign in question regarding metamask

I am curious as to why upon signing in with metamask , some other web3 helper (or call it whatever you want) like @usedapp/core only makes the user sign in to metamask , but moralis requires a signature’ . I need to further invesigate , I know I am a bit ignorant at this point, but I really like moralis, but like the way usedapp seems to just make you open metamask normally but moralis seems to require an additional signature. What transaction are they signing ?

Please correct me if I am wrong, and I appreciate your wisdom more than you could even know. If you have an address to donate to I would , you guys are so quick to reply.

This question is related to mobile or to a normal web browser with metamask plugin installed?

Firefox 92.0 from Imac mac OS BigSur (please don’t tell me it’s another BigSur headache lol?)

I’m guessing it’s because a regular sign in only returns the account address as far as I can see with used app so there must be some transaction to get all the metadata from metamask or something ;?\

I think that Moralis also creates a user in its database where it validates it with a signature.