Always.
But say, you decided to do a 60 minute dive, with an average depth of 60ft (15@100, 15@70, 15@50, 15@20), but you ended up spending a little more or less time at one or more of the depths for various reasons (octopus/wolf eels, photos, etc).
In my purpose, its mainly just an exercise of current profile, and for rough calculating average SAC for the dive. My downloaded profile will tell me my average depth later when I get home, but that doesn't help me on my second dive of the day, if the water is a little colder, faster current than I previously thought and trying to figure out if I have enough gas for a profile, or should I modify it so that I don't have to worry about running into issues with gas.
BRad