What's the benefit to pulse ox measuring during sleep?

It’s not used for anything else as far as I know.

I left it on during sleep the first few weeks just out of curiosity, but after confirming that the numbers were somewhat stable, I changed it to manual checks only.

Theoretically it’s used to detect sleep apnea. But you need to have an actually accurate pulse ox measurement device which most if not all wrist based devices are not. 

Well the idea is if you acclimatize in higher altitude for example, then the measurement will give you an indication for that. Sleep apnea might be a use case, too. But I wouldn't trust the sensor too much, especially because I loosen up the band a little to give my skin some freedom in the night. 

Theoretically it’s used to detect sleep apnea.

Yes, but the watch isn't doing that.

What the watch is doing is to estimate height acclimation, but I'm not sure whether 24/7 or sleep measurements are required for that or if manual spot checks will do it. The manual probably knows.

You get to see your pulse ox over time taken consistently at the same time and same conditions. Hard to do that manually.

It is used together with the Altitude Acclimation Widget to give you an indication if you have acclimated your saturation when changing altitude. If turned on during night it will give you a baseline of your saturation level. If you travel to a much higher altitude you will notice that your saturation will decrease compared to your baseline and once you are back to normal your body has adjusted to the new altitude.

https://support.garmin.com/en-US/?faq=cExJ3LSThbATTUqwvxzbm6

This is useful when the change in altitude is high, 1xxx meters or more, to understand why you don't perform as expected and to understand when you can start increase the intensity.

I don't get it really. With Fenix 6x I had the nighly Pulse OX on. In Epix to preserve the battery I turned it off. As I read in the link, in order to get the acclimation widget working, I would need all day pulse ox. Fenix 6X worked well with training altitude acclimation only with the nighly widget. 

Thus, what is the reason to sacrifice 2 days of battery life for nighly pulse ox? Or how are you doing it? 

I have it on during sleep, because with this watch you still get easily 11+ day of battery life with it on. At least what it shows me at the moment (10d at cca 70% of battery). For me personally it does not make difference if I have to charge it Day 12 or Day 14. However, I am not spoiled by Garmin's long battery life yet, as I was previous user of Apple and Samsung watches, which lasted (at best) 1-2 days :D

Ok, I understand your point of view. I switched from F6X which had 15d battery with nightly Pulse Ox. 

But still I don't understand, what do I lose by running Pulse Ox only on demand? What are the benefits for using pulse ox overnight or even 24x7? Does it affect other metrics? 

I don't use it because I dont have sleep apnea, don't train at high altitudes much right now, and it doesn't help with sleep quality metrics