I have watched a lot of frustrated beginning woodworkers attempt to saw a piece of wood while holding it against a workbench with their free hand or their knee. I even saw one diligent guy put a board on a workbench, then sit on the board while he tried to make a cut. Pity that all of them didn't clamp their work in a vise.
Vises are indispensable woodworking tools.
Through the day, a woodworker has to hold any number of things, such as parts or tools, so that he can work on them. Different styles of vises are made for a variety of different purposes. Thus, the longer you work wood, the more likely you are to own more than one vise. I started with one and now have six. A side vise is used for working the end or edge of a board.