An inside-out class can become a base class of any
Perl class (inside-out or not) without as much as a glance at its implementation.
That is the big deal about inside-out. Everything else is coincidental.
For a class author this means you can now publish absolute general-purpose classes. Every other class can simply inherit your methods and data. That is impossible with any of the traditional class implementations. It makes inheritance in Perl what it is in dedicated OO languages.
The tutorial is an excellent introduction to the use of Class::InsideOut, but as a general introduction to inside-out classes it misses the point.