An important property of a frame is the bracket stiffness.
This says something about how far the crankshaft resists, when you put force on the pedals.
With a limp frame the center of the bike leans back and forth and sometimes the rear tire to the rear fork and the chain may even run into the front derailleur.
This is at the expense of your precious energy.
Especially cyclists and mountain bikers benefit from the highest possible stiffness in the frame.
A frame with a high stiffness gives you the feeling that every step is hit, because no energy is lost in the movements of the frame.