Explained here
https://www.bt.com/help/email/bt-email-products
Without bothering to look up the history ... IIRC BT used to offer former customers a paid-for option to carry on using their email address if they left BT (which I think was relatively uncommon at the time). I think the above is a next-generation version of that.
Presumably aimed at those who have been with BT forever-and-a-day and have their email-world wrapped up in a BT address and can't be bothered to disentangle it to something else when they leave BT.
Haven't used BT email myself in decades but I regularly receive spoofed spam/scam emails from known BT email contacts/users whose details have been stolen in past data leaks. Descriptions from people I know who currently use BT email suggest it is on a par, or worse, than VM email.