Muscle range is usually better described as muscle extensibility. This is about muscles having the normal length so when they are passively stretched a normal length is achieved. Muscles that are too short may cause increased abnormal forces over time on a joint the muscle crosses. Muscles that are too long often behave as a weak muscle and cause similar changes in forces across a joint. Both these problems can result in soft tissue damage in the medium to long term and in the long term could be relate to degenerative changes in joints and soft tissues.
The above are examples of passive insufficiency (allowing too much or too little movement) Active insufficiency relates to the inability of the muscle to contract and work appropriately within a given activity. this is usually to do with weakness of the muscle although if the muscle is too strong or too stiff compared to other muscles this can also cause problems.
A large number of problems that one sees in physiotherapy relate to these underlying sorts of muscle imbalances - so yes this is quite important.