The Global Buyer List is rather useful and vendors aren't affected in any kind of way by this new feature. The access is solely for vendors, affiliates aren't able to access it and it serves more as a convenience for them.
It functions by an affiliate sending traffic, and anyone who buys through their link can now automatically be added to a new list in GetResponse (which is owned by the same affiliate who was sending the traffic), therefore buyers of the product can be segmented from the rest of their list(s) now.
In simplest terms, if a lead comes from an affiliate, the lead buys the product, then that buyer gets added to a new list belonging to the same affiliate. It is kept confidential and no other affiliates get access to that lead.
Below are a few examples of what this could be useful for:
1) Fulfill affiliate bonuses for the product.
2) Suppress the new list from additional promo mails for the product by the affiliate.
3) Segment a new "active buyer" list from a larger freebie list, etc.