If you are planning on using the entire contents of a video clip in a linear order you may still find it useful to split the clip, though you should be very sure before you perform this action. The main reason for this in your Windows Movie Maker project would be to alter one part of the clip while keeping the other clip clean or with different alterations. This is especially true if you are changing audio levels, audio balance between primary audio and secondary audio tracks, or a video effect. You may want to only use video effects on a split clip if you are splitting the clip along a perspective shift in the video clip.