Until you've learned how to light something, you really don't know if you're
choosing available light. No, a flash isn't required equipment. But depending on what kind of shooting you want to do, a flash can help you get the shot you want more easily. Lighting is more complex than shooting available light: you may need a lot more gear, and you have a whole helluva a lot more to think about. But you also have a lot more control.
But don't look at those Strobist photos and instantly assume all you need to make great shots is a speedlight.
Remember, you have to have mastered all the other photography bits, too. My word of advice is, if you aren't thoroughly comfortable shooting in Manual and swapping stops among your iso, aperture, and shutter speed settings, you may want to hold off on getting a flash until you are. Because when you add flash, you'll be juggling two more balls in the exposure mix (flash power & distance) as well as working on two different exposure levels (ambient vs. flash), and your shutter speed isn't going to have the effect you think it will.