Any modern copy machine will allow you to place a document on the scanner glass to make a copy. If you have multiple documents to scan, a document feeder like the DADF or Duplex Automatic Document Feeder will eliminate the tedium of scanning multiple documents and may allow you perform other tasks while the feeder does all the work.
Seeing both sides
The scanner beneath the DADF uses two image sensors positioned to record both sides of a document at the same time. Since the scanner can read both sides, scanning can be done in a single pass. Other designs using a single image sensor require the feeder to stop, reverse each piece of paper, and pass it back through the scanner a second time. A feeder using a straight through feed path is likely to have fewer mechanical problems in the future as well. There is simply less to go wrong.
Consider a DADF when you purchase or lease your next copier. It will cost a bit more but gains in efficiency and time saved are well worth the additional cost.