Ideally the zoom feature is useful when you want the user to be able to observe and appreciate intricate details that make a product attractive or useful. Though smashing magazine recommends extremely large product images to allow the user to understand the features and design of a product at a glance,
Either provide large images right on the product page or allow users
to click on an image to zoom in. You want users to be able to view the
image as large as is practical on an average monitor. This means an
image that enlarges to 1024×768 pixels is a good size to aim for.
large images might not scale too well the site design and hence the zoom feature allows the user to look into specific details which might not be visible in the general view
Another aspect is that since the shopping is online, for products such as shoes or clothes people cannot feel the texture and the ability to zoom in allows them to visualize the kind of material which is used to make the product. To quote this article on types of product images :
One of the downsides of e-commerce is that no matter how hard you work
on you product pages the customer will never be able to have the real
product in his hand, as you normally can when going to a physical
store. Therefore the customer can never feel the fine texture of the
silk cloth, the sturdiness of the titanium lock, or the precise
assembly quality of the camera. For some product categories (apparel
comes to mind) these tactile experiences is a major part of the
overall shopping experience and a decisive purchasing factor.
To close some of the gap between e-commerce and actually holding the
product in your hand, textural images can be used. Textural images are
extreme closeups that take the customer in so close that you can
clearly see the texture and assembly quality. In practice this is
often provided by either a zoom functionality for a
high-resolution product image, or it is a separate image in the
gallery providing a close-up of a part of the product.