Would you like an in-game parser?

[Mature]

Party composition has a very significant impact on your own dps. So if you want accuracy you need all the parties information not just a fraction of it

This.
Another clear example of personal DPS gain is Bard. Doing SSS V4s , a full 340 bis bard can pull up to 21/23sc left which is around 4.4k tops (without rng crits ,pots or food). But if you actually go into the fight you might be doing less damage BUT because of the party buffs you can pull a MUCH higher dps then your own SSS parsing.
Having the right party composition can increase your own DPS in huge proportions (using the same example with Bard, having a SCH,AST,DRG will make your Bard DPS increase in a ridiculous way)

But of course if we don't have a parsing , how do we know?

Another reason why we need a personal parsing.

Tbh without one there wouldn't be any guides in YouTube telling you when to execute certain CD etcs.