Watches are very rarely as waterproof as they say. There is a massive amount of fiddle room when it comes to exactly HOW water tight they are.
Computers give you essentially:
Dive time elapsed
Time until you need to make decompression stops.
If in decompression. What stops you need to make to offgas.
In my case my computer also shows, via a little wireless transmitter, the remaining contents in my tank.
Essentially.... It is telling you how to not die.
Now, I actually had a problem with a different brand.
With these computers they tend to decide your battery life is not allowed to go below 55%. After that the unit shuts down.
On a dive a few years ago I had the unit suddenly start displaying very odd content readings. 180bar suddenly dropped to 130 then 45 then back to 150 and the unit locked up. Dive aborted after 5 minutes.
I now have a seperate contents gauge as well.
However, you still need time and depth as well.
Especially in a cave in Mexico.
Safe diving all..