One reason is to measure your Cr, (recompression index)
You can also measure this from the first loadings step, where the line is pretty flat until it reaches its preconsolidation stress and then it will steep rapidly as its loading along the virgin compression curve.
I woujld add that some samples, espeically softer clays may have a good deal of disturbance, i.e., the loading curve will not be "flat, then drop" but will be "downward curved, then drop" . . . if you don't do a unload/reload you would likely estimate a greater Cr value than what it really is.
