I'd like to know why I would use a technology like Pointroll over
creating my own Flash ad that has the same functionality (for example,
an expandable banner or skyscraper)

Depending upon your skills as a flash ad creator, it is sometimes
better to leave flash creation to professionals. Most everyone knows
how to hammer a nail- doesn't men you want the 12 year old down the
street building your new house. Now if you are an expert coder, have
incredible computer graphic design ablities- then hey feel free to use
your own skills. Most people are willing to pay because they see their
personal limits on their ablity- or because they truely believe that
their time is best spent doing other tasks.

Just my 1.657 cents there though.