The automatic response of a muscle to stress is to tighten. Ideally, when the stress (pressure, pain, strain) passes, the muscle would then relax. However, prolonged stress or pain can cause a muscle to tighten and maintain the contraction indefinitely.
Muscles are anchored to bones by tendons. The fibers of a tendon, unlike a muscle, do not have the ability to lengthen and shorten. They are fixed. If muscles lose their flexibility, the danger of a tear or injury to a tendon increases greatly.
Ligaments attach bones to each other, and like tendons, they are tough and non-elastic. In essence, the entire body is reliant on muscles being pliable to avoid tears and strains. Muscle strength and tone are important, but flexibility is as important, and often overlooked. Relaxed and supple muscles can be used more fully than tight, tense muscles.
When I work on horses I often encounter muscle spasms: painful, involuntary muscle contraction either in the belly of the muscle or at the attachments. A muscle in spasm is unable to return to a neutral, relaxed, and supple state.
Regular body work can help shorten the recovery time between workouts and prevent tightening up and stiffening of muscles.