They are also useful for maintaining orbits (like the ISS), and could potentially be useful in long duration missions such as interplanetary transfers. If it takes months to get to Mars anyway, low thrust won’t matter if it means you get more total delta-v, and able to reduce the travel duration anyway, but of course this depends on the specifics

Yes, but when it comes to weight, it really is just calories in and calories out. A healthy diet is more than that though, indeed

You can still eat junk food and not go up in weight as long as you don’t overeat