Illustrator differs from Photoshop by concentrating on vector based drawing techniques but it is still invaluable in many design projects.
I use Illustrator extensively for logo design and branding due to the fact that they can scale at an infinite rate. It’s also a useful tool to create logos and icons because they can be scaled down to a minimal size.
Illustrator was used for the iPhone app project in many ways. The icons for each tool were created in Illustrator because they needed to be supplied at different scales for the different screens. For example, a double sized image is served to the retina display of iPhone 4 products.
In addition, Illustrator and it’s layers functionality was exploited in order to map London for the location based section of the app.