If your sport needs your shoe to bite in the ground as in cricket or golf, there are metal or rubber spikes on the outer sole. It also protects the midsole from stresses while playing or running. The midsole is a very important component of the shoe and is the main shock absorber. It provides the necessary resilience and reduces the incidence of injury in sport. The midsole, being the softest component of the shoe, very often fails the earliest.
Some persons continue to play with a collapsed midsole and land up with serious foot problems.
Often, you may have to discard a shoe which has not lost its serrations on the outer sole but which has lost the bounce of the midsole. This part of the shoe also has a cardboard lasting board which adds to its stability. This is the place where some manufacturers add the 'air sole' for extra bounce and comfort. The part of the sole which comes into contact with the foot is the insole . This is the soft and pliant lining on the inside of the shoe.