Because you want a new application to be supported by Accomplishments System (for example: a new game).
Because you want to create a new collection of accomplishments designed for your community.
See the Accomplishments wiki/documentation for more details, including how the accomplishments system recognises an accomplishment as being acheived (you write a Python script for each accomplishment). Note that most of the guidelines and ideas are centered around learning how to use Ubuntu rather than games.
Yes, you can create accomplishments for games. From The Guide to Creating Your Accomplishments (emphasis added):
See the Accomplishments wiki/documentation for more details, including how the accomplishments system recognises an accomplishment as being acheived (you write a Python script for each accomplishment). Note that most of the guidelines and ideas are centered around learning how to use Ubuntu rather than games.