Good catch! They are basically interchangeable, except that marked seams stay with the mesh, while selection unwraps don’t, so you’d need to select the same area every time which can be tedious. The one area where I like to use selection instead of (or in combination with) seams is when using the project from view style of unwrap. The advantage of that is it makes the UV’s look exactly like what you see in the 3d view while a regular unwrap is sometimes warped, but the disadvantage is that there can be stretching if you’re not careful.
