As we age, our joints and muscles continue to need motion to maintain their balance and motion. True for animals too. When your dog and cat (or other pets) begin to reach those golden years, daily exercise is even more important in some ways.
Joints begin to stiffen somewhat with arthritic type changes and that limits the amount and comfortableness of the joints in question. In dogs, it is very common to see that stiffness upon getting up first thing in the morning, after lying for the day, after too much vigorous activity. Like the rusty hinge we all know about, after we get things moving, they move easier and with less resistance.