It’s important to have your work piece held by some kind of vise when you’re working with it. It is really helpful in order to have both hands free to do everything that you need to do when you’re soldering.
A vise can allow us to manipulate the part in different orientations that makes it more convenient for us to work with. For example if we need access to both sides of the board we can tilt the board up on edge and have access with both hands from either side.
For some smaller boards it may be possible to grip it from the edges, and for that you might use a vise that has a wider jaw especially when you’re working with something that is very, very small.
And finally, for large parts, you run into the problem where the board begins to flex when you apply pressure, so in that case it’s often helpful to have two vises handy so that you can clamp it from both sides. In this case we have a lot more force that we can apply before the board bends and flexes. So sometimes it’s helpful on a larger board to have two vises.