This is my best attempt to explain this phenomenon. No doubt someone has studied this more than me, but here is how I see it:
I will highlight some things that you already know beginning with your very first workout..
- After intense use, your muscle has small tears.
- Your muscle urgently requests resources to begin repairing and rebuilding for the next intense use.
- Your muscle sends pain signals to your brain.
Now lets observe this, briefly, from an evolutionary perspective..
- In the past you needed your muscles to acquire food and resources in order to survive
- Pain is a negative reinforcement for "bad" behavior
When your muscle tears, and your muscle sends pain signals to your brain, it is effectively incentivizing you to rest that muscle. This prevents you from damaging your muscle even more, which takes much longer to repair, and causes you to lose muscle mass, which ultimately makes it more difficult to acquire food and resources (less muscle, less food right?).
In effect, the pain discourages you from weakening yourself.
Why does your muscle eventually stop sending pain signals to your brain?
- Your muscles have been able to repair themselves successfully several times
- Pain requires some attention and can be quite distracting
Your mind at this point realizes that your muscle is going to be used intensely repeatedly, and it will usually be given the proper amount of rest. And also at this point, your mind begins to drop or "ignore" the signals sent from your muscle (or muscle stops sending signals.. not 100% sure). This has some advantages from a survival standpoint:
- It is much easier to repair your home or prepare for a harvest without aching muscles
- It is much easier to, say store food for the winter, without aching muscles.
These tasks require minimal muscle usage compared to intense workouts and were paramount to survival for many people across the world. This ultimately helps you survive.
Basically, reduced muscle soreness helps you do your day-to-day stuff more effectively in a modern day scenario. In the past it helped you focus on staying alive.
P.S. I almost included that surviving a sudden bear attack would be hampered by sore muscles, however a different adaptation occurred that suppresses pain, increases strength, focus, and senses. This one is called adrenaline.