What is the point of it.
When you uv unwrap these will be edges which will be cut in the unwrap.
http://wiki.blender.org/index.php/Doc:2.5/Manual/Textures/UV/Unwrapping_a_Mesh
http://cgcookie.com/blender/2011/01/21/intro_uvmapping/
http://cgcookie.com/blender/2010/03/02/unwrapping-a-female-character/
When will I need to use mark seams.
When you want to control how your object unwraps such as to minimise stretching or give an unwrap that is easier to texture then if you use the automatic smart unwrap.