I don't know about other environments but when it comes to large (often open source) PHP projects that other people have written phpXRef is an absolute life saver (especially if the doc is placed online and Google can index it).
Even a badly commented project can at least help me track down where things have been defined and where they are used (for example when refactoring).
When well commented the resultant pages form close to a perfect Bible for the codebase (for my uses anyway).
Furthermore my prefered IDE will auto generate the comment block (if I type /**) which does roughly 75% of the commenting work for me. It is amazing how many stupid things I have been stopped from committing over my coder lifetime just because I have had to explain to other people (and future me) what I am doing. When my comment for the doc generator is bigger than the method this usually means I have not had enough coffee and might want to think a bit harder.
Those self same comment blocks also create the inline completion "help" text so I can see exactly what was expected by the other coders as I am writing the function call. This is a massive productivity boost for me (especially in those rare edge cases where some other helpful developer has written "for goodness sake do/do-not X" which can save a lot of pain.
I cannot stress enough how useful it is to have the expected input types specified in complex (and often badly named) PHP projects and the argument order in less frequently used methods. Even with my own code I cannot always remember what arguments I specified for something I've not touched in an age.
In one instance it meant that the reason for the recurrent problems was that for some reason that reflects badly on prior developers some functions and even constants were defined in a huge number of places (with a degree of inconsistency for added "fun"). That was the sign to walk away from the project.
So reasons include saving later developers a stack of time, keeping track of where functions are called (and defined), spotting silly coding, finding (as another has pointed out) when something is obviously missing, simplifying refactoring (never much fun) and in many cases getting an idea what the developer was trying to do (assuming he or she left some notes).
If the project is complex enough to have multiple licenses going on (no fun) I can quickly see which licenses apply to any given section. Admittedly this is a side bonus.
In larger projects which started before I joined in I can see which developer (assuming they tagged the class file with a name and email) created the class and simply being able to find and talk to the right developer is hugely helpful.
Automatic task lists - using the @todo tag (common in the kind of projects I find myself working in) means that the documentation can keep track of stuff that needs some more work (or features that are acknowledged to be missing). Again my IDE keeps track of this and that alone acts as a good guide as to what needs my attention first.
Also do not underestimate the value of keeping pointy-haired bosses happy at the touch of a button.
Lastly (and very important to me) it removes the non-trivial overhead of writing all that out and then trying to keep it up to date when some (read many) coders commit changes and don't talk to the documentation maintainers.
In short the "auto documentation comments" are vital to my coding habits. I am sure there are many who think that's lame but I am also just as sure that there are a fair few folk that know exactly what I am saying. I don't know how I survived before discovering phpXRef (and my favourite IDE).