This is often associated with 'getting big' or achieving a bodybuilder's physique. It is far wider and more important than that. Improving or maintaining functional muscle mass is vital for functioning in everyday living – it is our muscles that allow us to move.
Resistance training exercises specially improve muscular strength and endurance, and offer numerous everyday, functional benefits to the muscles (as well as the bones) beyond just increased strength or power, including:
Further, as we age, we naturally lose muscle mass. It is therefore vital at least to maintain muscle mass and to continue resistance work over the age of 40. However, functional muscle mass is clearly necessary whatever the age.