As for why we get older, it is helpful. It makes sure our genes continue to shuffle.
Though it's helpful, I doubt that's why we age. I think it has something to do with an inherent limitation in how we evolved. Our genes exhibit their traits not just against the backdrop of our physical environment, but against the backdrop of their respective neighbor genes. Our genes have pleiotropic effects (meaning, they don't just have a singular phenotypic effect, but potentially many). When you consider that we have a bunch of genes (around 40,000?) which all have complicated interweaving effects on one another, and you consider that it's lucky if a single given gene has a positive effect, then you're pretty damn fortunate that you're healthy at any given time.
As we know, genes' effects on us change as we age. Of course they're going to be good effects when we're young because they need to be; otherwise, we wouldn't be capable of reproducing. But as you get older, man, it's simple statistics. There's no great reason for those genes to continue to have great function in your old age, and so those changes which must occur, as they are part of our genetic mechanism, are going to be harmful. As you get older and older, you accumulate more and more damage until one of your organ systems collapse and you die.