spikes increase the friction with the ground hence reduces slipping on the ground..therefore a better a grip for the sportsmen and he is able to move faster n safer.
sportsmen uses shoes with spikes as they need to run very fast and skid in the fieldĀ spikes helps them to run fastĀ 
spikes also provide better grip on the ground thereby increasing the friction between the irregularities
shoes with spikes offer more friction because of more irregularities in the surface. This gives better grip to sportsmen while running.

Well the spikes are used because due to the ground's smoothness the sport's man may fall due to less friction if the spike shoes are taken they increase the friction. for example take a object in your hand you can hold it now apply oil to the hand now and try to hold the same object can do so ? this is all because of friction.