Basics of Field Work

So, why do archaeologists do field research? Contrary to public fantasy, it is not to find treasure to put into museums.

 Archaeology is the study of human cultural activities through the analysis of material remains. This means that ultimately, archaeology is about the information we can gain from past objects and not the objects themselves. Every archaeological feature, artifact and site has the potential to provide greater insight into a past culture, whether that be about what they ate, what they built, what they believed or so many more possibilities.

One of the earliest, and now unacceptable methods is just basic treasure hunting. The earliest 'archaeologists' such as Heinrich Schliemann (who discovered Troy) were basically glorified treasure hunters. They excavated for the sole purpose of getting cool and shiny treasure to show off. Most didn't care about the wealth of information that could have been gained from the non-shiny artifacts, and so destroyed so much information in their reckless search for treasure.
When archaeology began to be a more scientific approach in the early 1900s, archaeologists began to be more interested in the information contained in sites rather than just the shiny things. These field excavations were done to determine a historical or chronological series of event.
Over the last century, the reasons and methods for field work have changed quite a bit. Over time, more interest was placed on the meaning behind a site (rather than just its timeline) and how what was found could be used to test theories about ancient cultures. From there, questions began to be asked about what was the meaning behind sites and artifacts (not only for past peoples, but for modern ones as well).
A kind of field work that is often done today is different kinds of evaluative, or community, archaeology. These kinds of field work are undertaken on behalf of (generally) non-archaeological parties, and are being done for specific reasons (such as to answer specific research questions).
Another kind of common archaeology done frequently can be called salvage archaeology - this kind of field work in undertaken in areas that are undergoing development of some kind and the archaeological finds need to recorded and/or preserved before the development can be completed. These kinds of archaeological field work can be easily seen in larger cities that have had occupation for many centuries.