The problem with this old-fashioned notion of capitalism is that it works only if value-creation comes from labor: you buy leather and make shoes, then sell them. You have a profit motive to make shoes, and if you work hard making shoes, you get money.
The problem is that the economy has been shifting for a long time away from that. If I have capital, I can buy a robot to make the shoes for me. If robot maintenance is cheap, then I can create value for almost nothing. Then I earn a lot of money, for doing nothing other than investing my capital. So, my ex-employees starve. (And then I have no one to sell shoes to! Oh no! Recession!)
Large pots of capital are growing larger, which centralizes wealth. We need a way of spreading this wealth around so that everyone can live. There are lots of ways to do the taking (capital gains, inheritance taxes, etc etc) and lots of ways of spreading it around (food stamps, social security, etc etc). UBI proposal is one of the latter, but it's important in the magnitude that it suggest is neccessary.