To maximise DoT damage, efficient re-casting is key. You should certainly check out guides for your specific class to ensure you're using the right method, whether that's clipping -- replacing a DoT before it's expired -- or allowing something like a mage Living Bomb to explode. It's often useful to be able to start the cast of a DoT to time it with its expiration, and of course, to maintain maximum DoT uptime for the minimum of GCDs and mana, and for all those reasons, you need to track your DoTs.
What the heck? Why am I starting with a nameplate addon? Well, dear reader, I understand your confusion, but for PvP purposes, Tidy Plates, or any similar addon such as the built-in ElvUI nameplates, is a pretty decent way to track your DoTs.