Bioanalysis is the study and gathering of plants and DNA material for use in medicine and consumption. In the Star Wars universe, most of the medical technology comes in the form of Kolto, a bacteria that will help heal wounds if you soak in it long enough, but other plants have similar restorative properties.
As you might expect, plant nodes exist throughout the worlds of SWtoR pretty frequently and will give you materials to apply to your Biochem profession. Diplomacy is the other crew skill that provides mats for this crafting profession, making them the most useful trio for one character. Missions make for quick leveling of the profession, but are often costly and you can do most of your work in the field leaving your crew free to work at crafting and diplomacy.
The most useful crew skill while leveling (outside of Slicing) is Boichem, as it provides you a group of easily usable med kits and stims that will aid in your leveling (as well as a easily salable item on the GTN). As such, Bioanalysis is one of the few skills worth leveling up planet to planet while you are powerleveling a character to 50.