While using a VU meter is not necessary for a clear mix. Giving a proper headroom is still relevant today.
The VU meter is modeled on the old VU meters where the copper needle was purposely made ‘weighted and calibrated’ to create a “rise time” of around 300ms.
This may be better understood with a little bit of biology.
The way I understand the anatomy of the ear . The earbones need between 66 to100 miliseconds to perceive a sound. In some elder folks that number rises to around 300 ms. Which means ears react differently with age. Which is why the older VU meters were calibrated that way. It is true that human ear needs a bit of “reaction time” to understand what is going on.
VU meter plug is just emulating that to make sure that your clipping levels stay within ‘audible human reflex’ times. Real vs Perceived loudness has to do with biology. Which is why we say trust the ears.
anyway… for digital headroom why I still leave headroom is to avoid inter sample clipping along the road.
If I am recording 24 bit 96khz, I tend to leave 2db headroom so in case my mastering engineer uses analog converters, chance of intersample clipping is less. If I am recording 16 bit 44.1 I tend to leave more like 5 db, this really depends on your sample rate. The faster your sample rate, the less headroom you need. This image will explain a bit. The bars in red are digital samples, and if you throw it in an analog converter, the converter will perceive a higher peak than your digital peak.
In short, leaving headroom is sort of “backwards compatibility” and “scalability” in software terms. Its not for you, its for what your sound will undergo in the future. Like ending up in some funky remix…