One bag is over the limit and another has room. This works out precisely how much to move, and what the fee would have cost you.
One bag is over, another has room. This works out exactly how much to move, and what the fee would have been if you had not.
You are at the desk, the scale reads 54 lb against a 50 lb limit, and there is a queue behind you. The agent will happily charge you an overweight fee, which on most carriers is between $100 and $200 for a single bag. Moving four pounds into the other case costs nothing.
The arithmetic is trivial and completely unavailable to you in that moment, which is the only reason this tool exists.
If the total weight of your bags exceeds the total allowance across them, no amount of shuffling helps and the tool says so rather than pretending otherwise. At that point your options are to move weight into a cabin bag, wear the heaviest items, or pay for one overweight bag — usually cheaper than paying for an extra bag outright.
Airport scales are not calibrated to the pound and yours at home certainly is not. A bag that reads 49.5 lb in your hallway can read 50.5 lb at the desk. Aim two pounds under.