There are other, lesser reasons, of course, including: to prove you can do it, to compensate for failings in other things, to impress others, to leave a legacy, to help perpetuate your genes, or maybe because you just can't think of something better to do with your time.
But I'll put aside those lesser motives, which I tend to view as psychological foibles. Basically, money gives you the freedom to do what you'd like – and when, how, and with whom you prefer to do it. Money allows you to have things and do things and can even assist you to be something you want to be. Unfortunately, money is a chimera in today's world and will wind up savaging billions in the years to come.
As you know, I believe we're well into what I call The Greater Depression. A lot of people believe we're in a recovery now; I think, from a long-term point of view, that is total nonsense. We're just in the eye of the hurricane and will soon be moving into the other side of the storm. But it will be far more severe than what we saw in 2008 and 2009 and will last quite a while – perhaps for many years, depending on how stupidly the government acts.