AAC (Augmentive Alternate Communication) software that I know of (The Grid, Gus!, SofType, Dasher, Click-N-Type, Keystrokes, etc) gives the user access to an on-screen keyboard (with various kinds of word prediction, abbreviations, etc) and redirects these chosen keystrokes into another application such as MS Word, Excel or Explorer. There is also some facility to enable clicking and dragging the mouse. The main aim is to give the user with a disability full access to normal, standard, able-bodied programs. This is a good thing - as far as it goes. The problem with this approach is that these normal programs are not always that easily accessed even by the able-bodied - notwithstanding Microsoft's extensive user testing! Although the modern graphical user interface is second nature to the youth many of an older generation have difficulty with it. Have you ever tried teaching your grandmother how to navigate a web browser? Explaining it fully is actually quite complicated and not instantly grokked (keyboard focus, keyword search vs url, single/double click, scrolling options, etc.).
Sue Center, instead, creates a world (albeit quite limited) within which the user with a disability can 'live' - not needing to leave that world to use normal programs. This world is tailor-made with the limitations of the user in mind.
An analogy that may make this difference clearer:
A person who is very short has difficulty living in a normal house because all the counters and tables are too high. One solution (most AAC software) is to put step ladders everywhere or to wear elevator shoes. Such an arrangment is perilous because one can fall from ladders and elevator shoes are teetery. Another solution (Sue Center) is to redesign the house lowering all surfaces to a more optimal height.