If you're giving away software at no cost then you are taking advantage of the marginal cost of software being close to zero.
You could charge, but then you might fail in the market due to competitors not charging. You can attempt to build a market for your time, by giving away the software.
Giving away software can definitely build a market for your time - but in the end you're selling your time, which you have a limited amount of, instead of a software product which might scale indefinitely (see "Angry Birds", for example).
You could charge, but then you might fail in the market due to competitors not charging. You can attempt to build a market for your time, by giving away the software.