It lets users merge two or more workouts into a single one. There have been times when I have been out riding, hiking or whatever and accidentally end the activity on my apple watch instead of pausing it. Starting a new workout means having your stats split across the two workouts.
The "usual" way to merge such workouts is to export all of them to individual FIT files, then use a tool like fitfiletools.com to merge the individual FIT files. You then have a merged FIT file, which is difficult to import back into Apple Health. This process also requires access to the internet, which is not always guaranteed when out in remote areas.
MergeFit makes this process easy by merging workouts right on device and without the need to deal with FIT files at all. It reads data directly from Apple Health and writes the merged data back to Apple Health.
The app reached a small milestone a few days ago - crossing 1000$ in total sales.
If I downgrade myself to a non-admin user, and install apps into my home Applications, then I'm not bothered by permissions requests from apps to update themselves. Almost all of them can just do it, on their own, with non-admin permissions. The only exceptions I've found are Tailscale and other stuff that needs higher level OS integration.
Edit since upvotes: Non-admin user operation was recommended by the Pareto Security app, see info on this specific item: https://paretosecurity.com/mac/checks/not-using-admin
All Pareto security checks: https://paretosecurity.com/mac/checks
App: https://paretosecurity.com/mac and https://github.com/paretoSecurity/pareto-mac