Blades have to be pulled very tight to work. A loose blade would wobble too badly to cut anything. Blade tension is measured in pounds per square inch. Cutting blades have to be pulled somewhat more tightly than slitting blades.
A saw should include a blade tension gauge that allows one to adjust the blade tension when installing new blades accurately. If blade tension is too low, a cut will be of low quality. If blade tension is unnecessarily high, there will be more wear and tear on the equipment.