Muscle Fiber Density allows a soldier to reach elevated positions without the need of climbing aids. Indirectly it allows a soldier to travel further especially in alien ships when there are many elevated positions.
The gene mod is especially useful for Snipers with Squadsight to move into elevated firing positions. The gene mod is also useful for soldiers to enter into elevated cover in order to reduce the chances of getting hit by an enemy.
In Covert Operations, this gene mod is recommended with a Sniper at Colonel rank with Low Profile and Mimetic Skin; this combo also proves extremely useful as most missions have high points and high cover.