The cheapest fare on the page is rarely the amount you end up paying, and checked baggage is usually the biggest reason. On unbundled carriers the bag is priced as its own product, it costs more the later you add it, and it costs most of all at the airport desk. Turning up with only a cabin bag keeps a low fare low.
What it buys beyond the fee
You skip bag drop and the carousel, which on a short trip can be most of the time you spend in airports at all. Nothing can be mislaid by an airline that never took it. And on a connection, there is nothing to collect and re-check, which is exactly the failure that turns a tight transfer into a missed flight.
Making a week fit
Pack a small set of clothes that all work together instead of packing options, roll rather than fold, wear the heaviest shoes and jacket, decant liquids into small bottles, and plan one wash for anything over a few days. Almost every overweight bag is a bag packed for a version of the trip that did not happen.
Check the rules for your carrier, not for carriers generally
Cabin bag limits vary by airline and change more often than you would expect, and several low-cost carriers now sell the overhead-locker bag separately from the small bag that fits under the seat. Read the two dimensions and the weight for the airline you actually booked, then compare fares as totals including whichever allowance you will really use.
