Rarely not never
Hola
As usual cjb had raised a good question.
I wrote that "I rarely use snorkel" that means sometimes I use it.
When I use it:
Training: when I'm doing surface swimming
Sea or lake: For surface swimming or breath up with bad time.
The rest of the time I don't use it.
For example, in dynamic apnea or static apnea I breath-up without it.
In Constant Ballast training (specially in lake) I prefer to lie down face up, for breath-up. But the weather has to be calm, because with many waves is hard to breath-up face up. The roll down can be done very smoothly, with little effort, and is easiest to pack without the snorkel (it can be done with it, but I don't like it).
Despite the breath-up (face up or snorkel) I always throw away the snorkel at the beginning of the dive. I never liked the movement of it, in the way up, and it's better to exhale before surfacing, than to blow an snorkel.
When I'm freediving for UW photography, always use a snorkel because it's more comfortable, and the freediving range is more conservative.
The snorkel is more dead space, but in a rough sea is the best way to breath-up.
I hope I had clarified the concept
Sincerely
Frank Pernett