Hmmm can't say ever removed the cap either. I thought the pressure would find it's way back to the reservoir tank in the engine rising the level?
You say pop a seal how would you know?
Well if you pop a seal on you caliper you brakes won't work and you will have brake fluid coming out the dust cover
Also as for letting air into the system your not letting any air in your just allowing a non pressurised path for the fluid to return back to the reservoir.
Everyone may not do it like this but that's how I was shown to do it properly, by a mechanic