Rolls are mostly used as a means of evasion (e.g. dodging an enemy's attack) or a breakfall after falling from height to reduce the impact on hitting the ground and keep movement going.
Occasionally, rolls can be used for moving swiftly at a low height for getting under an obstacle, or to get from one spot to another while staying out of sight as much as possible (this is used quite a lot in the Splinter Cell series).
Uses in Real Life
A common real life activity that rolls are used in is parkour, as practitioners often face large drops, and rolls keep the forward movement going fluidly.