One of the great advantages of the Thai bag is that it bends nice with thrusting or power kicks, and as I said, that movement can help with timing and judging distance. One of the important aspects of kicking the bag is to develop power, but it also helps with targeting, as I usually pick a spot and pinpoint where I am kicking.
Someone holding mitts does this better, but often times we don't always have the luxury of a partner being there. If you have a good Thai bag, you can do different drills such as this to refine the kick in other areas aside from power.
Here is a short clip of me doing ax kick and jump spinning back kick, sorry the quality of the video isn't good, as it cuts out in the middle, but these are the kinds of drills I like to do for timing and to have a target to hit, and they of course can be done alone if you have a heavy bag.
http://www.youtube.com/watch?v=HKwC2bG5ZtU