If you're in the middle of a spell cast or CA cast while your autoattack is ready, it has to wait until you are done casting for it to fire. So if your autoattack is a pretty big hit (and for most scouts and such i'd say it is) you can increase your dps by timing your CAs and such so that you maximize the number of autoattacks (ie minimize the amount of time that they're "waiting" for you to finish casting).
A visual bar makes it that much simpler to do.
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