Identical Twins?

For the same reason a doctor uses different types of scissors or scalpels for specific cases, or a handyman carries different types of screwdrivers, a smart designer will have the tools that are necessary to complete any design task that might arise. In many ways Photoshop and Illustrator seem to be exactly alike, but they aren't.

Golding, Mordy. Illustrator World (2005). Articles>Graphic Design>Software