Here's the science.

https://www.huffpost.com/entry/body-on-exercise-what-happens-infographic_n_3838293

You can also look online for many studies on how exercise is beneficial to the body even if you are overweight/healthy weight

Keeps you from getting overweight wich is bad for your heart and meny other things in your body.

Regulates blood pressure

It helps regulate your energy ither if you have to much or to little.

Some people it helps them sleep

It can help with stress

And probly meny more bennifets

One reason could be that aerobic exercise causes damage due to the formation of reactive oxygen species (ROS) which damage DNA and other cellular structures.

The body compensates by upregulating repair enzymes which are then maintained at a higher concentration (even when are you not exercising). Since way more time is spent not exercising than exercising, the net effect is more protection from oxidative stress.

This podcast gives the best answer to this that I’ve ever heard. IHerman Pontzer’s interview about research on this topic made me rethink my sedentary lifestyle.

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it does a bunch of things to a bunch of different tissues and systems--that's not a really helpful answer, but if you're wondering about mechanisms of action you'll have to specify what's being acted on. the physiology of tendon adaptation is different from the mechanisms that help lower blood pressure chronically