APT stands for Almost Plain Text. It's a simple document format that uses "formatting hints" to transform a plain text file into a fully formatted document.
A parser analyzes the document and looks for "formatting hints", such as indentation for paragraphs and stars (*) for lists, and generates a formatted version of that document into other formats such as HTML, DocBook and PDF. In other words, you write your documentation in text files without thinking much about formatting, and APT Editor does the rest.
Because APT offers features that other formats don't: easy comparison between versions (ever tried to compare two MS-Word documents?) and easy reading and editing (isn't reading XML like reading a web page by browsing its source file?) just to name a few.
Because APT has one major drawback: you need to convert from one format (plain text) to another (ex: HTML) to view it formatted. Also, in order to have a formatted to consult, you need to make the conversion every time you make a change and manage several files. With APT Editor, all you need is a single .apt file and you make a conversion only when it's really needed outside of your IDE (for example, to share the file with other departments or post it to your website).