When you have added additional profile fields. It is much more user friendly for our buddypress website when users can change everything on one page. Rather than have an extra page just to change their email.
And another reason is that with xprofile users can have several email addresses. Such as the .edu school email, their private email and even their work email for alumni. By using the default email setting page, buddypress only allows users to change 1 of them. While with xprofile you can have all 3 on one page.
I am not a programmer, but I can image that if a plugin syncs usersnames it should not be a very big change to also sync email or any other field.