@Alex-Parfenov
Agreed that NPP is not an IDE, but, once one adds the proper plug-ins to it, these are some of the things that can be done with it:
Compile and link programs and jump to errors in the source code (NPP Exec)
Tag and use tags on source files (TagsJump and TagsView)
Keep many sets of files (sessions) at hand for editing (Session Manager)
Easily compare editor files to files in source control and check in files (Tortoise SVN)
Open files by name/pattern that are part of a solution (Solution Hub)
I suspect that a fair portion of long-time NPP users are programmers of one sort or another, and for good reason: it has a lot of features we use (syntax highlighting and the like) and it supports plug-ins that allow it to function pretty much like an IDE.