Gravity applies a practically constant force on you at earths surface. If you don’t go deeeep underground or really high above earth, 9.8 meters per second is a pretty good number.

So, on earth’s surface, one kg of mass is approximately 2.2 lbs of weight and 9.8 newtons.

Go somewhere where gravity is half as much as ours and the same kg weighs 1.1 lbs and 4.9 newtons.

Force is only worthwhile as a PROXY for mass when measuring on equal gravity. So, for most experiments on a single planet with a strong gravity, it’s fine to use force for calculations.

Someone here says they’re 400 lbs and you may be able to picture their fitness level. Someone on the sun’s gravity weighs 400 lbs and they’re the size of a baby. A 100 kg person weighs differently on the sun too, 30 times more, but they would still be 100kg. And dead, but this is a thought experiment.

When you’ve got multiple gravitational fields to think about, knowing the mass of an object makes your life way easier. Even in the US, physicists use metric for this very reason.

How did I do for simplicity?