Most of the general public considers those of us who exercise to be slightly off-center. They can't see the benefits, only the effort expended.
There are loads of technical reasons why "sweaters" sweat, but the simple reasons are the ABCs of fitness -- endurance, muscle strength and flexibility. There's nothing fancy about that group. There are interesting reasons for working on these three items.
Endurance can point the way to a healthy heart.
That means your heart muscle works better, slower, steadier and returns to lower rates when you slow down. It's called recovery rate, and it is VERY important for those stressful moments when the kids drop light bulbs from the second story windows, or when the boss finds out you lost the alpha file. Cardiovascular training will enable your heart to pump more blood with each stroke, thus getting more oxygen to your whole body when you need it the most.
Building muscle strength is a simple, easy way to prevent musculoskeletal problems. Think of it this way. Your muscles and ligaments are the glue that hold you together. If you don't have strong muscles around your skeletal frame, you'll flop. Your mind will tell your legs to move, but unless you have introduced your leg muscles to your brain, you're going nowhere."
One more item to consider when you look at your wimpy muscles; the bones that these muscles and ligaments are attached to pull your bones back and forth. And (especially under weight-bearing conditions), those bones will thicken and become stronger with the movement. When you slip on the ice next winter, you may not break anything.
Flexibility is a word that most fitness types don't have in their vocabulary.
Unless they are hard core exercisers, they will leave before cool-down or look the other way when toes have to be touched. Consider this. If you run, jog or use your legs during bicycle movements, you will build strong, tight, short hamstrings. That's from bending the leg up in back in the same muscle group usage for hours at a time. Now you have to work the opposing muscles in the front of the leg and stretch that hamstring to even up the muscle balance before you tear a hamstring. That's why you see runners spending time with each leg up on a bench and in a lean-down, weight-bearing position before and after they run. Flexibility has a lot of benefits. Besides, it is nice to occasionally greet your toes without having to lie on the bed to do so.
These three exercise reasons are basic, easy reasons to take up fitness. There are loads of other side benefits, but total body health comes first. It won't work if you try for stress relief without getting oxygen benefits from a conditioned heart. And you need your muscle strength to maintain proper posture. Finally, you'll need flexibility before you can walk with grace. Simple reasons for staying fit.