Simple. Store everything. I give you an example: the clay tablets written in cuneiform discovered at Ur. They were disposed of by Sumerians, possibly because th

Simple. Store everything. I give you an example: the clay tablets written in cuneiform discovered at Ur. They were disposed of by Sumerians, possibly because they have fulfilled their purpose and were simply thrown away. They deal with important things like commercial transactions, but also unimportant things like a personal letter or a poem. This unimportant things pretty much taught all we know about the Sumerian language: syntax, vocabulary, regional variations, etc. In archaeological terms, a refuse is a huge treasure trove, precisely because no one chose what to throw away. Everything is there. It's entirely up to the archaeologist to comb and come up with an interpretation.