Actually, we are interested in the soil itself, in addition to any objects that may be in it in situ. Archaeologists are interested in intact soil that has formed over the millennea by natural processes. Its the matrix of soil that tells the story about seeds, strata, charcoal deposits, pollen samples, the arrangement of rocks used to build a fire, arrow heads and ceramics. Once the soil is disturbed, anything it may have contained is of little or no archaeological interest. The provenience is destroyed. The integrity is lost. Most of what archaeologists do in an excavation is shaving soil profiles, to read what is there and interpret the sequence of deposition in that particular place. The layers provide the context for interpreting c 14 dates, and the sequence of occupations. It is the information in intact soil that archaeologists are after. Everything that is removed in an archaeological excavation is meticulously labeled with a provenience location referencing soil profiles to identify exactly where it was in relation to the soil matrix.
