It limits the negative effects of evolution (which, I might point out, includes both genetic and environmental/social mutations). Remember that evolution is random, it does not have any direction. And in particular, it has no requirement to head toward an optimum, only cull certain negative mutations.

Some negative mutations have a direct personal effect - a physical deformity, or a mental failure to hide from a predator. But others have an indirect, social, or delayed effect - imagine, say, an individual who failed to collect food for the group to survive winter, or even raided the stores instead of collecting when food was common. It is clearly in the group's best interest to punish that individual - either by excluding the individual from specific group activities, or even outcasting or attacking. Afterwards, the contrary individual will either conform or perish.

Remember that fwd-fwd-fwd-re-fwd story about the 5 monkeys and the banana on the ladder, where they would get sprayed if one of the monkeys went for the banana, so they beat him up even after replacing the monkeys one by one? It is often used (correctly) as a story warning that we don't understand why we perform social punishments. But it is also a demonstration of evolution's protections working "as designed". None of the monkeys had any reason to believe that anything had changed to negate the need for the social punishment - and remember, evolution has no motive to be perfect.

As a higher-thinking species, we can of course ask ourselves whether any specific social punishment is actually giving any benefit. Since our social behavior is learned over millions of years but our history only lasts thousands, we cannot of course remember where they all come from (if there even is a single cause). But we must realize that every single one of them has some cause, either a problem that we still face today or else a problem that we (or some subgroup of humans at least) faced in the past.

The internet is often stated to be a place that you can be anonymous. And yet, all internet cultures spontaneously enforce a very strict social rule: you do not change your identity. If you take some action under one pseudonym, you cannot just start using another to escape punishment or make it seem like your position has more support. If you have some affiliation with a company, you must not promote that company or degrade its competitors without revealing your affiliation (but if you do reveal your affiliation, there is no punishment for holding your position, although it does allow you to be punished for other reasons).

This all comes from a variety of sources, but what made me start to put it all together was reading an article called something like "Why do birds sing?".