We use it because it is a better IDE. I am a professional who gets paid for my output and I pay for tools that enable me to get more work done faster and with fewer errors. If PyCharm saves me one hour every year over other alternative editors it will have paid for itself. I assure you that it pays for itself every year.
Eh? Do you mean pycharm? Even so what's wrong with a professional product being paid-for and closed source? Developers have to eat too, many of which I'd imagine also contribute to other opensource projects, using... pycharm.