Are they needed?
Most dive boats require them on all dives. IF you don’t own one, they will usually have them available to rent.
But why not own your own? It is one of the pieces of equipment you rely on to keep you safe, literally keep you alive, so maybe you should own one so you have the peace of mind that it is in perfect working order and you are accustomed to using it.
Features and price come with experience. Start simple and move up when your needs and dive level necessitate it.
Computers have batteries, batteries can fail or the computer fails from flooding unexpectedly. Tech divers usually have a “Backup” analog gauge just in case, consider that.
The “Hoseless” computers use a transmitter on the 1st stage to send pressure to the computer (usually on your wrist). Again transmitters need maintaining or they will fail. Hoseless computers are wonderful in that they eliminate that one extra hose, tell you a ton of information, store your past dives, etc. I have used hoseless for as long as they have been available, BUT again I have a small analog backup pressure gauge. IF you can afford hoseless, we recommend them.
Hosed computers are also wonderful in that they tell you tons of needed info, BUT if you have one use a lanyard so it does not bang into things when hanging down AND spend the small amount extra for one that detaches from the hose (with what is called a Quick Release) to store it, take with you on a plane (not in baggage, which goofs up their calibration at times) and when needed on the boat so it is not knocking around with your tank and BCD.
Our advice is free if you want to discuss the plus and minus’ of Models, Brands, expense, etc.