IMHO the whole point is your body also doesn't really know how fast it needs to breathe. Often when we're stressed we're taking fast, shallow breaths even if there's no real lion in the vicinity. By telling your body to breathe at X speed, you break the stress cycle

YMMV

This is kind of the basis of an entire field of yoga called Pranayama. Its translation varies, but can be roughly translated as breath control.

A lot of it is breathing slower, or faster than your body requires (including retention of breath, or holding with no air in the lungs), in order to elicit mental and physical states.

Another example of this is Wim Hof, where you over breathe intentionally and then hold the breath for extended periods.

Breath is a really interesting topic, being an autonomous system for the most part, but one which we can take voluntary control over.

If you are interested in learning more, I'd recommend the book "Breath: The New Science of a Lost Art" by James Nestor.

Edit: After trying the meditation, I also found it was a little quick if I was trying to sync my breathing to the dot. It may be intentional if the aim is to improve focus and alertness for an upcoming task, rather than relaxation!

You can breathe deeper or shallower to control that.

Maybe that's the point. Because you have to follow a certain timing, your body is not on autopilot anymore, so you have to be conscious of your oxygen needs, maybe even adjust them (for example, by calming down).

Thank you. Yes. I do too. I had asthma pretty bad as a child, and rate-limiting my air intake, or really in any way trying to ensure that my breathing is meeting some spec, is super stressful for me.