The problem with running an app on your Mac, is that you will only see the bandwidth used by that machine, not the other devices in your house, such as a Roku box or other Macs, iPads, or streaming devices. The router occupies a unique network position to be able to see every packet going to and from the Internet over your ISP connection, by every device.
My router doesn't have any breakdown either, but it does have a "reset counter" button, so you can check how much you are using over a set period of time by resetting it at intervals. I just use a calendar reminder to reset it at the first of the month, and even then forget to sometimes. Once you have a rough idea, you'll probably find it doesn't vary too awful much from month to month, and the idea of just "knowing" gets old for me. I did find that our average usage went up when we upgraded our Internet service to a different provider with much higher bandwidth (went from 7 to 30Mbps), but even then, it settled into a boring same-old average, just at a higher level.