Makes sense for three reasons:
1. Google can give visibility of the capabilities of Gmail to non-Gmail users and, hopefully, use that as a showcase to persuade users that the grass is greener if they switch to Gmail.
2. Google gets access to email content that they wouldn't have normally seen - which can then be sent to their servers for data mining.
3. Users who want a consolidated inbox split across multiple common email providers can do so, without having to have separate mail apps (which, in cases such as OEM variants, don't work as well). Those users who don't, can continue to have separate mail apps.