It is the product of a literate community that had the access, the resources, and the desire to create a written record. The archaeological record is a bit more democratic. In this sense archaeology is a great equalizer. Archaeology tells untold histories, especially those of the daily lives of regular people. The archaeological record does have its own biases.
Preservation is one.
Perhaps more problematic, however, is that the majority of what archaeologists study are things that have been discarded, whether it is an abandoned house or a trash pile. Pompeii is a rare example where a natural disaster captured a moment in time. From the bits left behind, archaeologists set out to reconstruct a culture. Documents of all sorts, written and pictorial, are integral components of archaeological research at every step of any project. Some uses are practical and straightforward and form the basis of any good research design.