A parser itself isn't needed to clear the hardest tiers of content. With that being said, having a parser readily available can provide feedback for players to see what they are and are not doing correctly.

IE - Two DRGs queue into Sephirot (Story) together. Both have similar HP values so you can imagine they have similar ilvls but at the end of the fight, one DRG is at 1.4K DPS and the other is a 900 DPS. That's a pretty big difference so someone can start to ask "why is the weaker DRG weaker?". This can range from not knowing how to play the job optimally, multiple deaths, unfamiliarity with mechanics resulting in lower DPS, same ilvl left side pieces but dramatically different (or even incorrect) right side pieces, etc. Or even a combination of all the above.

The number itself means little without context hence why it's not entirely necessary to clear content. With that being said it certainly gives players red flag if something is amiss compared to equally geared jobs or something is correct if the numbers match up nicely.

Sadly, something that an ingame parser will most likely be unable to do is interact with FFLogs, which is a tool that basically de-compiles your ACT parse in a very readable way that allows parties to analyze all the details of their fights to see where they can and cannot improve - which is a great benefit for both progression parties or farm parties as they try to find optimizations in their performances.

So, is a parser necessary? Not in the slightest. However it does provide a lot of feedback that can be useful to many players as long as they understand the context behind the number.

And now. *Gets the popcorn for the drama to ensue*