And it also allows for some leeway in profits. If the assumptions regarding the milestones are wrong, or there are cost over-runs, then net income has to be adjusted. I don't know when companies must do this "catch-up", but if they are good, then the smoothing happens early and isn't put off until the end of the year in a surprise adjustment.
Of course, the reverse could be true as well (i.e. underestimating profits), but my guess is that this is a much less frequent occurrence.