The law in some areas requires it and there is a fine for violations. In some dive environments it is a good tool. And finally being an Instructor I have to set an example.
However, some folks absolutely refuse to see the value in a snorkel and they wouldn't be caught dead with one on the side of their head. If you personally feel this way but live in an area where a snorkel is enforced, then you might benefit from one of the fold-up styles that pack tightly in a BC pocket.
It's quite funny, I've seen lifeguards in Laguna Beach observe divers apparently entering the water without a snorkel. They stop and ask them to see their snorkels and divers pull out the fold-up kind. What is so funny is these snorkels are often in their original packaging with rusted staples through the plastic and cardboard goop that once was used in the store to hang the snorkel on a sales rack! These guys are doing the bare minimum to meet requirements.
I wear a dry snorkel on the left side of my mask. I teach with it and as a habit I use it all the time for surface swimming. It's second nature now, so the only time I notice it and actually wish I didn't have it on is riding a DPV at full throttle - when the little bugger whips violently back and forth against the side of my head. You know ... now that I think of it I really ought to go and get one of those fold up kind <lol>