hello i am new to dos i usually play as a tank i was just wondering are dots a must?

[Allyrion]

For any given class or job, the ones native to that class/job will be essential for maximum dps. So BRD needs Windbiter and Venomous Bite, DRG needs Chaos Thrust and Phlebotemize, etc. There are some weird levels as you're leveling up where individual dots might not be worth it, but as a rule you want to maintain dots at all times (unless the enemy will die really fast).

^ This for the most part.
But I'd say 30 second dots have a very niche spot thoughtout the leveling process. It will usually need to last maybe 21 seconds (varies individually) for it to be effective.
They find some use in Dungeons on off targets, but mostly good for Bosses.

Shorter Dots are really good, especially BRD dots - Venombite is really short and Windbite procs Bloodletter.
For potency comparisons, you can divide the time by 3 (the server tick interval) and multiply it by the potency.

For example, Mutilate starts at 18 seconds. It does 30 potency per tick.
You could say it gains 30 potency every 3 seconds then, and at 18 seconds it's 240 potency.
It also delivers 60 potency on-hit, so it's good for 300 potency in total.

You can compare this to your other moves and their potency and see how many ticks it needs to surpass them.
In general, they have a very high potency and that's why all the innate dots of a Job are worth it. They just take some time to do that damage, so if the mob dies early it's a waste.