It emerged to tackle the problem that image sizes and resolution was growing faster than internet bandwidth - so it was difficult to quickly browse high quality images in web browsers.
IIP allows the detail to be fetched when the user needs it, so the whole data file is not downloaded before.
Practically it defines how software fetches image tiles from a server.
Zooming and panning are carried out by fetching higher resolution image tiles from server.
This means any size of image can be viewed on the Web without downloading it all.