Who says it has to be good? We cannot assume that every trait that evolves is beneficial to the species. A lot of people assume that the species of an ecosystem will evolve optimally; that is, they assume each species evolves to have the best possible genes for its environment. My understanding is this is not the case. Species have to adapt or face extinction, but they only have to adapt to be good enough. Perhaps it's not that forgetting is good, but rather it just doesn't impede survival or reproduction. (Well, except when you forget your spouse's anniversary!) Which leads us to the next point...
It's a side effect of how human memory works. First off, storing a memory in the brain is not like running a tape recorder or camera. The brain actually stores very little of the information coming into it, just enough so that it can later fill in the blanks and make a decent reproduction. 759
In computer terms, the data is filtered through a very lossy compression algorithm before being stored. It's just good enough that we usually don't notice, but sometimes things go wrong and we get phenomena like confabulation.
Now about how that data is stored. As you know, the data is stored by the synapses in the neurons. But here's the thing: those synapses aren't dedicated to that specific memory! Storing a new memory affects synapses that are part of other memories. One synapse might be storing a piece of information for any number of memories, because those memories all have some tiny thing in common. I wish I could explain this better and in more detail, but this became pretty obvious to me when I read up on artificial neural networks, which are crude models of brains.
Sometimes, neurons just die. Obviously this is going to affect the brain's ability to recall the information the dead synapses contained. There's enough redundancy built into the brain that this is rarely a problem, except in cases like Alzheimer's, where the brain just suffers too much damage.
To me, the question isn't why we forget. The question is how we can remember stuff at all! 77