Yeast can clean up some fermentation by-products.
This is from From 'Yeast: the Practical Guide to Beer Fermentation' (by Chris White and Jamil Zainasheff), which is well worth purchasing and reading.
Beer matures in the stationary phase, also known as the conditioning phase. Yeast reabsorb much of the diacetyl and acetaldehyde produced during fermentation......
........One of the things you do not want to do is force the yeast into dormancy before they have had every opportunity to clean up after themselves.......
........Our recommendation, especially for homebrewers, is to wait for the yeast to finish their tasks and clean up fermentation by-products as much as possible.
What you can do to speed this up, is improve and control fermentation conditions as much as possible. If you aren't getting many unwanted by-products produced in your beer, there isn't much for the yeast to clean up, so you can package earlier. If you properly control pitch rate and vitality, oxygenation and ferment temperature the wait for clean up is very short!