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Not sure [why] really.
Yes, it will ask you for an email address. I think one reason is to be assured that the AppSumo 'member' is not 'cheating.' They may also send you an invitation to join the group.
That is the reason I suggested using an email alias. My mistake, even though I meant to include that in my OP, I obviously failed to do so! Using an alias helps to know when that specific address has been sold/traded/stolen because you usually know who you gave it to. But it is also easy to delete/change and block any more emails to it.
Most ISP's provide at least five free alias' and I would encourage everyone use as many as they have available. Never use your main address for anything but your "friends" who also run AV software on their Windows machines.
FWIW, I used my gmail addy and so far I've not received any email from these folks.