Partly, it is for variety.
Also, if you want to focus on strengthening/increasing muscle in the biceps and triceps, you might find that they respond better putting them together on their own, rather than always at the end of a larger body part. For example, if you always work biceps after back, you may always be a little tired when you get to biceps. Also, the biceps get some work when you train back, so you may not be able to lift as heavy for biceps in a back/biceps split. The same goes for triceps, which get some work in many chest and shoulder exercises.
On the other hand, working chest/triceps/shoulders together, and working back/biceps together is good idea if you want to do these workouts on consecutive days (eg Monday and Tuesday). If you were to work chest/back on Monday, you shouldn't really work biceps/triceps (or shoulders) on Tuesday because the arm muscles (and shoulders) would also need to rest from the chest/back work.
Hope this helps,
Sharon