if rem is directly proportional to px, what advantage does it possibly hold
The answer is that it isn't directly proportional. It depends on the font size. The formulas and converter work under the assumption of default font size.
It's like those measuring cups that have kg and liters, do you have one in your kitchen? They only measure liters, so the kg is a misleading lie. But because most kitchen-liquids have density close to that of water, it's a sort-of-useful trick anyway.
Same with rem and px: most users will be using 16px = 1rem so it's a good starting point, but the whole reason rem is better is because sometimes 1rem != 16px.