One of the main use-cases is air integration, which allows you to have a transmitter connected to your cylinder which monitors gas pressure and relays it to the dive computer on your wrist.

This is an alternative (or, depending on the school of thought, in addition) to having an SPG (submersible pressure gauge) which is hopefully attached to your left d-ring, but for many recreational divers stuffed in a pocket somewhere or scraping on the bottom.

Air Integration is quite an investment (3-500€ per transmitter) and there is basically no interoperability between brands.

When Garmin came out with this, it made quite a splash, as all of a sudden instructors could keep an eye on the cylinder pressure of their students. This is a very minor use-case though, and I don’t believe it’s used a lot.

As for underwater communication, a good chunk of the scuba community is quite opposed to relying on electronics for communication. Hand-based signals are quite good, underwater paper does exist, and if you’re so far from your buddy that you need to wirelessly communicate you’re in a lot of trouble.

This all being said, as a boat captain responsible for the people underwater, I would love a local area network that allows me to pull the plug on a dive, or get some emergency notification. For example “weather is turning, please call the dive” or “I was blown off the wreck, please pick me up”.