David,
ArchiCAD is a program specifically designed to assist in the practice of architecture. It also has applications in other aspects of building design and engineering, urban planning, interiors, landscaping, etc.
The program by itself is only useful if you have a need for it. The tone of your post suggests that you are not (yet?) a design professional. This is probably a prerequisite to really get much out of the program.
If you are thinking about getting into the field, it is very rewarding and endlessly interesting, but VERY much work and no so remunerative as medicine or law. In short, you have to love the work, but for the right people it is easy to love and the hard work is worth it.