I was always told the lower class or poor are there to scare the middle class into working and supporting the upper class.
The middle is the padding/margin that keeps the possibly of moving up and out of your class a seeming reality, but also keeps the divide from rich/poor from being too strong. If you just have rich/poor, the opportunities seem more dire, less opportunity and less mobility is possible and lots of bad things start to happen.
A society that tends to the middle is a more healthy one, one that tends solely to rich/poor needs leaves those middle people out of luck, that is what we have now.
This is also a consumer economy, the lower/middle spend over $1 or 100%+ of their incremental dollar earned, the rich only spend .05 or 5% of their incremental dollar. It is good to have consumers that can spend in a consumer economy, it helps all levels of class/wealth. Rich/poor do not spend enough to keep a consumer economy afloat that actually moves money the way it is supposed to move and you end up with stagnation and long drawn out recessions. In downturns, the poor have no money and the rich don't spend or invest in downturns enough, the middle always has to spend.
If you sell products it is also nicer to have middle class out there that can buy, not just upper class. The products that target each are fairly different and there are only so many rich consumers.