Crude Extracts
To being any sort of purification procedure you need to obtain the material from which you plan the isolate the material. Historically the abundance and ease of isolation dictated which proteins were first studied (e.g. hemoglobin). Also many proteins are common to a large number of species (e.g. metabolic enzymes) so they could be isolated in large abundance from other sources, such as yeast or bovine.
Once you have gathered the material containing the protein you want to study it is necessary to generate a crude extract -- for proteins from muscle that would mean grinding it up, for an intercellular protein that would mean breaking the cells open, etc. This is always done in the presence of a buffer and inhibitors. Why? As a scientist you want to control the environment -- keeping the protein you are interested in at a non-denaturing pH, you want to keep it from being cleaved by enzymes that will be released in this process so general inhibitors will be important, etc.