I used Mail on my One M7 for quite a while, and it's nothing fancy. The app pulls down your messages via IMAP or POP3 like a traditional email client. In terms of features, you have the essentials. You can manage multiple accounts, organize messages into folders, format text, and search through your inbox. The reason I used it had little to do with any advantages over Gmail. I just liked how Mail blended in with other Sense apps.
That, to me, remains the most compelling reason to use HTC Mail. But it's also a good option for non-Gmail users in general. In addition to Google's service, HTC Mail works with Outlook, Yahoo, and Exchange.
This version of the app is incompatible with my M7, so presumably you need one of the company's newer flagships to use the app and receive future updates.