https://finance.yahoo.com/news/fitbit-versa-lite-review-193641289.html

The Versa Lite is more or less a clone of the original Versa smartwatch, which came out last March. The main difference you'll notice between the two devices is that the Versa Lite only has one button, while the Versa and Versa Special Edition have three; one on their left side and two on their right. The missing buttons let you launch and interact with certain apps.

You're not actually losing any functionality, though, since you can perform the same actions using the Versa Lite's touch screen controls.