The problem with the tables is that they assume you spend your whole dive at your deepest depth. Unless you are diving wrecks from a boat, you will rarely do a profile like that. It is very difficult to create tables that will allow you to assess a dive or plan one that is really multi-leveled (the PADI Wheel was an attempt at this). Computers do an iterative calculation of nitrogen loading as you dive, and compare that to the algorithm they have been programmed with, to see where you fit. Computers therefore take into account the time you spend in shallower water, and therefore will almost always allow you more bottom time than tables.
However, it is very good to have some framework on which to hang your dive planning and your computer readout. If you know, for example, the rule of 120 (that your depth plus your allowable bottom time equals roughly 120), then if you are at 80 feet and your computer is showing you 60 minutes of remaining NDL time, you might question how well it is working. Of course, none of us is doing 60 minutes at 80 feet on a single tank, not even me! Especially for new divers who are following the guidelines for their certification level, gas supply will loom much larger than decompression as a limiting factor for dive time. It's always puzzled me how OW classes spend tons of time on tables and DCS, and little or no time on gas planning . . .
