Great question i definitely considered going fully client-side at first to avoid backend costs and "risks" lets say.

I choose to keep a server backend for a few key reasons:

- Centralized updates to the exercise database (videos, translations, attributes) without shipping a new frontend each time. (The goal is to have 5 to 10 new exercices per week)

- The ability to offer shared features like community workouts, saved routines, and (eventually) syncing across devices, etC...

Room to grow into more advanced features like progression tracking, public/privates APIs (strava, garmin, ...), and integrations all of which are much harder to manage purely client-side in my humble opinion.