My first reason to “shred” documents (that is to look closely at each statement a source makes) was that it forces you to be aware of the details.
The second reason to shred your documents is that each bit of information in them has a different chance of being correct. If you lump each individual statement from a source together, you may either end up disregarding the whole source just because you disprove one of its statements or accepting that the whole document is correct simply because part of it makes sense to you. Often sources record a mix of firsthand and secondhand information, recent events and half-forgotten happenings. Those differences are important; they can mean the difference between fact and fallacy. Taking a good look at the individual statements within a source can allow you to concentrate on whether each one in turn was recorded based on the knowledge of a witness when the information was fresh or was it already ancient hearsay by the time it was put down on paper. A mother’s statements to a census taker about her own children may seem reasonable and be correct without implying that her statements about her mother-in-law, her husband, their farmhand or even herself have the same accuracy.
So, the second reason to “shred” your documents is to allow you to get a sense of the truthfulness of each statement on its own merit.