Well let's start by noting that this is not always the case and that there are exceptions to every rule. But the vast majority of product photos that you see on the average ecommerce website depict the product on a plain white background. The overwhelming number of requests that I get are for just such photos.
In my opinion there are four closely related reasons to do it this way.
1. Simplicity - product photos that you see on an individual product page of a website are informational in nature. The primary purpose of these photos is to inform the customer about the product. So it is usually good practice to present the product in its most simple and straight-forward form. A plain white background eliminates clutter and simply presents - and presents simply - the product.
2. Focus & Emphasis - By presenting the product on a uniform, uni-color background, we naturally drawn attention to it. The product is clearly the most important part of the image, and for that moment at least, the most important thing in the potential customer's mind.
3. Contrast - Contrast is defined as "the difference in luminance and/or color that makes an object (or its representation) distinguishable." A white background usually allows for creating maximum contrast between the product and background. But consider the request I get at least once a week: to photograph a white object on a white background. In the most extreme case, the product becomes indistinguishable from the background. Luckily, there are a couple of ways to deal with the issue.
The first way is to define the edges of the product. This is usually done by manipulating lighting and/or reflections on the product. A very simple but effective technique is to use black cards to remove light from the edges of the product (this is commonly known as "negative fill"). This darkens the boundary between the product and background and allows the product to stand-out.
The second way is to exploit the fact that humans don't really see pure white. Rather we see shades of gray. So a very effective way to create contrast is to reduce the brightness of either the product or the background resulting in one being more or less "white" than the other. The product will still be perceived as white even if it is actually a lighter or darker shade of gray.
4. Neutrality - The final reason to choose a white (or perhaps light gray) background is that white is neutral in terms of color. While using a colored background can often be dramatic and quite interesting visually, that color is often reflected back onto the product itself. This bleeding of the background color can distort the true color of your product. What few people realize is that white is not the absence of color but actually the blending of all colors. This means white brings out the natural color that is already there.
So there you have it: White is often the best background choice when it comes to creating product photos.
Or is it? As I mentioned earlier there are exceptions to every rule. So next time we'll look at the question of "Why Not White?"