Trimming is useful to fine-tune the timing of your edits. Editing is all about fractions of a second. A cut a second (or even frames) too early or too late can affect how your viewer perceives a sequence. As an editor, you often need to watch how your shots flow in the Timeline.
Sometimes this fine-tuning involves changing the moment a cut happens; sometimes it is determining whether a shot needs to just end sooner or last longer. Other times, a shot might be perfect, like a reaction shot, but it happens too soon or too late in the sequence of events. In Final Cut Pro, you can use a variety of techniques, such as Ripple edits, Roll edits, Slip edits, and Slide edits, to trim Timeline clips and edit points.