The point is that many writers cannot image (read) the disc 1:1 which is required for making a perfect backup. A reader is a CD, DVD-ROM or burner that can image 1:1.
For example, Acers cannot 1:1 LibCrypt protected PSX discs (nor can they write them) nor can they image a SecuROM protected title because they can’t read the subchannels where the protection is stored.
A lot of Samsungs can’t read subs either for example. If you are stuck with a particular model that reads like a stuck pig but writes anything you throw at it then it may pay to invest in something like a Toshiba or LiteOn DVD that can skip protection errors rather quickly, read subs & play movies.
If your burner is an all-rounder like the LTR-24102B or PX-W2410A then one can live with one device only.
The salient points of reading are:
1:1 is the aim
Error Skip - wanna do Max Payne in 4’ or 4 hours? Wanna wait 40’ or 40 hours for LaserLock?
Error Correction - image data must not be corrected
SecuROM - gotta be able to read 16 bytes of PQ sub channel data
LibCrypt - gotta be able to read 96 bytes of P-W sub channel data
Hope this helps