Falling is the most important thing to master in
parkour, because mastering good falling and landing
techniques can lead to less injury.
Landing properly reduces the impact and force applied onto
the body by redirecting it into your forward velocity.
http://s1.hubimg.com/u/4904070_f520.jpg
Landing with a forward roll takes some force in the
downward direction, and moves it into a force in the
direction of the roll.
This is used to keep your velocity up while reducing the
impact on your body because it increases the time of the
impact, which lowers the overall impact.