When I was in college, I hated banks. Once I bought a cup of coffee for $2 only to find I had racked up $40 worth of NSF fees accidentally. I'm sure this happens to poor folks all the time. Poor people often don't use banks because banks prey on the poor and often make them poorer.
Why didn't I just use cash? Because I wanted to buy things online, accept payments from others, and didn't want to carry cash around all the time.
If Bitcoin was less volatile, it would actually do a good job of solving some of the issues faced by poorer "underbanked" people.