Just variety, different characteristics will appeal to different people.
Peak horsepower isn't the whole picture.
The 500 motor might be smoother than the tractor engine in the 700.
The 500 might be cheaper?
By the same token, why do Honda market seven 125s in the UK?

Only a year ago, Honda had a big gap in their motorcycle range from 125 to 600 and nothing in between, now they have plugged that with CBR250 and the new 500's.
They must see the market changing both at home and abroad.

The market is changing, partly because of licence changes and partly because people are wanting cheaper more economical and practical bikes and scooters.
Should also point out that Honda have announced that the UK will get the new for 2013 NSS300 Forza in May.