Why do musical instruments have to be tuned? You see in some concerts that the musicians stop in between to tune their instruments. Why?

Sounds from musical instruments are based on their timbre or quality, loudness and pitch. These factors depend on the beats, standing wave patterns and resonance produced while playing these instruments. These factors in turn depend on the tension of the strings, lengths of air columns, reflection from the cavity within the instruments, etc. All these vary while playing – especially since concerts are played in the glare of high intense stage and spotlights. Due to heating effects as well as the friction generated while playing, the parameters start varying. These makes the musical instruments go off tune from time to time. This is the reason why you see musicians tune their musical instruments from time to time.