Marking seams basically tells Blender how and where to divide the face groups. So, if you have a cube, obviously you would want to mark every edge. This tells Blender to seperate each of the six faces from each other on the UV map based on those seams.
So it really depends of the model you have as to where to mark the seams. If you have a cylinder, you would want to mark the outside edges of the top and bottom faces, and mark a single edge along the cylinder's depth. This will tell Blender to map the top, bottom, and outside faces of the cylinder as seperate groups.
Though, you don't have to mark seams to unwrap a model. You can use do it manually by selecting multiple faces from a mesh and then press U to unwrap only the faces selected. You can also chose "project from bounds" when unwrapping, and it will unwrap the faces you have selected based on the view your looking at them from. Hope that helps
Kravenwolf