Breakdowns are generally used to adjust/tweak animation timing.
You can convert channels of keyframes to breakdowns. What makes breakdowns different then keys is that when you scale the timeframe, breakdowns automatically scale the values of the curves so that it porportionaly keeps the same motion data.
If not you'll tend to throw animation out of sync or have other curve funkyness as you scale keys in an animation and have key values go through the roof [usually a reason that I key in all linear/stepped at first to nail down my posing and my timing then only add spline/flat curves where I need to adjust motion/arcs].
All this info is found under help btw.. they give a few examples under the animation section of the html manual.