Edith Cowan University research revealed that muscle strength benefits are seen with consistent, short exercise sessions. Participants performing a three-second eccentric bicep contraction thrice weekly saw strength improvements. Daily 20-minute exercises might be more beneficial than a single 2-hour weekly session. Regularity, even in short durations, is key to health benefits.
Evolutionarily, it makes sense if you're pushing a muscle to 100% exertion, even for only a few seconds a week.
Otherwise, animals would have to spend a large amount of energy to maintain or increase muscle mass, which is wasteful and inefficient — the species who needed more energy to maintain muscles are likely extinct or limited in number.
There's another element to this. Muscle tissue is metabolically expensive, so it's beneficial for an organism to limit muscle mass to only as much as it needs to succeed, thus reducing how much food is necessary. There's actually a protein, myostatin, that directly works to inhibit muscle growth. Some specific breeds of cow lack this; search up Belgian Blue cattle for a look.
Yes. The key difference is the near 100% exertion. If the muscles are used to they're maximum on a regular basis, the body will consider them necessary for survival.
If you suddenly dropped the weight by 20%, so that you exert those muscles less, you would expect them to gradually weaken by a similar margin over time; eventually, to the point that lifting the 80% weight would require near 100% exertion.