This test is designed to help you determine your T-Pace, or your Threshold Pace.
Some coaches like to use a CSS/CV (Critical Swim Speed)/(Critical Velocity) test, which is much shorter. The CSS/CV testing is great for determining your current threshold for distances up to 1,500-1,650m.
For longer distances, such as 70.3 Half-Ironman and 140.6 Full-Ironman distances, I prefer to use this 20 Minute test (and the 30 Minute version). I believe these longer tests more accurately reflect your fitness capabilities over a longer distance course.