Healthy (and unhealthy) fats are important for building muscle and for maintaining life for heaps of reasons.
-Good source of energy that digests slower and packs more calories per gram than protein or carbs.
-Healthy fats 'clean up' bad fats in your system and help combat against heart disease among other things.
-Fats are essential for building hormones in your system, especially sex hormones like Test., without which you wouldn't be building muscle.
-Many, many other essential processes in your system rely on fats to operate effectively.