In article , lid
says...

How does this actually help?

It reduces the overall forces on the booster.

The aerodynamic forces on the craft are a function of its speed and air
density. There's not much one can do about the speed, given that the
craft is accelerating.

True.

If the craft accelerates more slowly overall, for each speed it will be
traversing less dense air than it would do otherwise. But in that case,
is there a benefit from high initial acceleration and then reducing
acceleration on the approach to max-q?

Forces acting on the booster are from gravity, air drag, and thrust.
During max-Q, air drag is at a maximum. To keep the overall forces on
the booster below the maximum operating limit that they've set, they
reduce thrust during the period of time that it is experiencing maximum
dynamic pressure.

Is it really about reducing vibration loads during max-q?

That's one of those "devil is in the details" things. It's my
understanding that they're trying to keep the overall forces down.
Vibration might be a contributing factor though. But, that's something
only a SpaceX engineer would know.

Note that the space shuttle did exactly the same thing. You might be
able to do a bit of Googling and find more details for the space
shuttle. For example:

Mission Control Answers Your Questions
Question: What is the maximum dynamic pressure, and why does the
shuttle throttle down at max-q?
https://tinyurl.com/yydc968v

Jeff
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