I used a dive computer once, an Aladin Pro and hated it, dive ruined by constant beepings and warnings, spent so much time looking at the %^**&$ screen I saw nothing else.
After that I just used my training, dive tables and common sense.
I've used an Aladin Pro either as a main computer or as a backup for just about all my dives. A dive computer records exactly what depth you* are at in the water at any given time in the dive.
IME the Aladin Pro (like all computers) will nag you with beeps to warn you of ascents in excess of the recommended rate and if you are getting constant beeping then (assuming that you are not shooting to the surface like a Saturn V) you must be yo yo-ing in the water. The dive tables assume you are diving to a predetermined depth (or series of decreasing depths) and sticking to it and that any ascents are made at a steady rate. Constant yo yo-ing even by a few meters during a dive increases the risk of the build up of nitrogen micro bubbles in the body tissues and means that you are not diving the profile you and the tables assume you are.
In practice, to minimise the risks of "the bends", burst lungs etc. both the tables and computers are pretty conservative - the tables assume that divers (particularly novice ones), do not dive perfect profiles and restrict depth and times accordingly and computers "punish" poor diving skills (especially rapid ascents), with annoying beeps, longer decompression or safety stops and if you are diving again within the next 24-48 hours, a shorter dive time next time out, to reflect the extra nitrogen micro-bubbles in your system.
It can be quite humbling to download your dives onto a computer and review the profile you actually dived to the one you thought you were diving (you might choose to ignore that bit where you went up a bit quick but came down again and carried on but the computer won't) and my diving skills certainly improved using a computer and with a little practice, those nagging beeps just disappear and you can get in with enjoying the dive.
I get much less opportunities these days but my passion is/was underwater photography and I have been very happy to rely on dive computers as the primary way of letting me get in with what I enjoy doing during extended trips where I want to spend as much time as possible in the water. I do now usually dive with a second computer and I'd like to think would have enough experience to recognise a malfunction which was letting me letting me dive deeper or longer than I should.
Anyway, far too much if a ramble about diving on a bushcraft forum and if you think that bushcrafters sometimes focus too much on kit rather than the experience itself, trust me divers and in particular underwater photographers can take that habit to greater depths.
* If diving in a drysuit with a cuff dump valve, wearing a dive computer on the same wrist is not recommended as every time you raise your arm to vent the suit, the computer thinks that your whole body has shot up 2' in the water - with annoying results.