My niche use-case: I have four young kids and a minivan with a DVD player for them. I've used it to rip DVDs that we own and burn just the main tracks to blank discs. That way the movie will automatically play, and play on a loop, when we insert the disc instead of going to a menu screen and requiring interaction with the remote control (which my wife and I can't do from the front seats, and my kids couldn't do in the early years). It also means that we handle the original DVDs much less often, so they don't wear out; I can always burn a new one when the DVD-Rs get scratched up.

Lots of platforms have a maximum file share size.

For instance, if you want to post a video to Discord, you have a ~50MB cap unless you pay like $10/mo

I use Handbrake to compress videos over that cap. I could post it to YouTube but I don't want to clutter up my account with hundreds of 20-30 second uploads.

YouTube will re-encode videos to optimize their playback experience. Which means there will be a generational loss in the re-encoding. YT have maximum requirements for their playback. If the source file exceed those, they will lower it.

Handbrake is useful for lots of things. My partner have to upload his dissertation defense video and the university have strict requirement of the formats. He used Handbrake to convert his video to their formats of choice. It works well for him on his aging 10 years old MacBook Pro (upgraded the HDD to SSD years ago).

An example is if you have a DVD as an ISO file and want to transcode that to a different format. HandBrake is more convenient for this since it handles ISOs natively, whereas with ffmpeg you will need to mount that ISO and operate on the VOBs manually.

I am not a handbrake user but not everyone want to upload their vids to youtube.