Click to enlarge gives the customer a better and close-up look of your product details, different angles, color choices etc, while keeping the customer on the item page.
Customers interested in purchasing your products will surely want to get a good look at what they are purchasing before they check out. For this reason Yahoo! Store has built this functionality into your store’s standard features.
But when a customer clicks on the default Yahoo! Store image, they are taken to a new page displaying the full size image that you have uploaded into your editor.
One small caveot is that on the Yahoo! Store setup there is nothing to advise customers that they have this ability. They must assume or find it by chance. Further, while going to this new page may help your customer to make an informed purchase, leaving the item page could allow your customer to lose interest or to get sidetracked.
Example Store: http://www.idigitalcamerastore.com/can-4925b001.html