Personal dive computers (PDCs) have revolutionized diving by making information available that was not available before during a dive. That is because a PDC can sense and process information continuously and display it for the diver. A PDC can also warn a diver of dangerous situations.
There are basic PDCs and very advanced ones with numerous features. However, they all do one very important thing: They use depth (ppO2 for the rebreather diver) and time information to keep track of nitrogen dissolved into your body via a mathematical model, and they display how much time you have remaining without acquiring decompression obligations.
Gas-integrated PDCs also calculate remaining air pressure and, based on that, remaining dive time. Dive tables calculate allowable dive profiles only on an assumed square profile (that is, a dive that goes straight down to a set depth, remains there for a set time, and then ascends directly to the surface). Because a PDC continually monitors depth and surface intervals every few seconds, the PDC is far more accurate. The PDC stores the diver's actual profile for future reference.
There are PDCs for open circuit divers that can be switched over to provide information to the rebreather diver when diving closed circuit. These PDCs are growing in numbers, although at present, they are limited to a small section of the dive industry.