Only just stumbled on this after realising that a client was correct. They set the Profile to use Email (home) but the Contribution page overrides this and makes the email be Billing.
This ticket seems closest to the issue though it is initially about the idea of being able to collect additional email addresses.
If John K is correct then current approach is
If the contact is logged in this field defaults to the 'Billing' email, if set, otherwise the 'primary' email
When the contribution is confirmed the email address is saved/updated on the contact as the 'Billing' email
(ie: If the field was prefilled from the primary 'home' email, the contact would now have two email addresses the same for 'home' and 'billing')