In Brief: Evolution

Evolution isn't about "good" or "bad".  It is simply a word for a specific emergent process. It describes all the things that happened that led to the current state of affairs.  Sometimes they happened for reasons, under specific and identifiable pressures, but other times just by accident. It gets way over-simplified, especially by people looking for answers, since evolution is bad at providing answers, or reasons, because it's a description of an emergent system and not a driving force.

The original building block of evolution were these two observations:
1. Does a trait make it likely you will die early or otherwise won't reproduce when someone else will, given current evolutionary pressures? Then a trait is likely to be expressed in very few members of a species.

2. Does it directly lead you to have more kids, given the current environmental pressures? Then a trait is likely to spread, being is expressed in a larger percentage of the following generation of a population that contain that trait. "Being able to digest milk when food is scarce" is a good and recent example. Note that even this doesn't imply a value judgement unless you think humans' value is based on reproduction (which some evolutionists do because they're wrong.) People who can't digest milk aren't defective. Indeed, environmental pressures can change and which traits are adaptive will change with them: now that we have better nutrition being able to process lactose may no longer be an evolutionary advantage.

It turns out that in addition to the two more obvious dynamics, there are a bunch of other cases too:

3. Is a trait situationally useful, sometimes helpful and sometimes not?  It's likely to show up in some of the population, but not most. (There is an interesting cluster of traits that occur with 8-15% prevalence in humans, including male pattern baldness and ADHD.)  This is similar to a mixed equilibrium in game theory.

4. Does the trait allow for on-the-fly adaptation?  As programmers, we know how powerful reconfiguration can be.  The human brain, for example, is highly plastic and can adapt to changing circumstances, and our muscles grow better at performing exactly the tasks we perform with them.  Specialization is "expensive", in that it leaves the organism vulnerable to changes in the environment; allowing for cultural, technological or physical adaptation during a lifetime is an easier way to get a similar effect.

5. It is a trait that was once useful or is useful for some people even if not for you, and is not actively harmful? It may stick around! Dimorphism is complicated to evolve and thus usually only occurs under pressure.  This is why women have a prostate and men have nipples. Once something has evolved, it takes pressure to make it go away entirely, which is why we go through a phase in utero when we develop proto-gills.

6. Is it fun/attractive/entertaining/not actively annoying? Then it may not contribute to inherent fitness, but it is likely to be selected for anyway, because evolution isn't a passive thing done to us. It is a dialectic process: the process shaped us, and we get to shape the process. Cultural tastes or norms can lead to evolutionary pressure just a surely as any other environmental factor (which is how the Hapsburg's lasted as long as they did: cultural power was more influential than any pressures against genetic disorders.)  This is similar to mechanism design in game theory: if we don't like the outcome of the game, change the game.

7. Is it genetically linked with something that is subject to any of the other positive dynamics?  Even if a trait itself is not useful or desirable or advantageous, it may share a common cause with something that is.  

8. Finally, does a trait have no reliable impact on reproductive success? Then it might happen anyway! This is called "genetic drift". Sometimes answer to "why?" is "eh, why not?"

Assuming that something is one of the first two may seem really cool, but when trying to impress your friends and intimidate your enemies always remember: a trait might just not be bad enough to be worth getting rid of.