An engine is not a good comparison to a body. Engines do not have self-repair mechanisms. Bodies do. Engines do not degrade if they don't run. Bodies do.

Regular moderate exercise improves muscle tone, boosts immune function[1], increases bone density[2], and extends life by increasing cardiovascular health[3]. It increases neurogenesis in the brain[4][5].

Lack of exercise, on the other hand, is associated with poor heart health[6], diabetes[7], and other serious medical problems[8].

[1] Exercise and immunity: MedlinePlus Medical Encyclopedia

[2] Osteoporosis Exercise for Strong Bones - National Osteoporosis Foundation

[3] Regular Exercise Critical for Heart Health, Longevity - American College of Cardiology

[4] https://www.sciencedaily.com/releases/2016/02/160208083606.htm

[5] More Proof That Aerobic Exercise Can Make Your Brain Bigger

[6] Exercise and the Risk of Coronary Artery Disease

[7] Preventing Diabetes

[8] Risks of Physical Inactivity