Before dive computers became popular and widely used, divers calculated their NDLs using dive tables. Dive tables base calculations on the deepest dive point that you will reach during the dive. They provide you with a conservative estimate of the allowable time you have for that dive (NDL) based on time, depth, and any buildup from previous dives in the last 12 hours.
Although the dive tables help keep you safe, there is a downside to using them. It is unlikely you will stay at the deepest point on the dive for the whole time. It is much more likely (and safer) for you to descend to the deepest point first and to gradually ascend as the dive progresses. This means that your air supply will last longer, your final ascent to the surface will be shorter, and you are exposed to less pressure at the end of the dive. The dive tables simply can’t calculate those changes in depth. That is why most people lean toward using dive computers.