The side kick is a powerful straight line kick executed by turning your body sideways and thrusting the leg outwards so that the striking foot is parallel to the ground from toe to heel.
One advantage of the side kick is that it is the longest reaching strike (outside of flying kicks), making it an excellent weapon for attacking from a distance or to counter an opponents offense as a stop hit.
Unless you fight from a very bladded stance, a disadvantage of the side kick is that you you to move out of your stance and into a sideways position to execute the kick. This means it may take a little longer to execute, be more conspicuous, and take longer to recover and return to your stance. This moving out and back into your stance is the reason there aren't as many combination options with the side kick.
I exclusively use the lead lead side kick as the rear leg side kick takes too long to execute.
Stop Hit Side Kick *example *Holly Holm example
https://www.youtube.com/watch?v=XSfqGkoUkqQ