Consumers love selection. If they are going to spend their time looking for something online, they have an expectation they will find it (seems simple right?). How many times have you driven to 2+ stores on a weekend looking for a product to not find it.
Online retailers are able to offer orders of magnitude broader selection than brick and mortar retailers. A very important concept called the long-tail, comes into play. There's an excellent book by Chris Anderson available here, that I always recommend everyone in e-commerce make sure they read. Chris also has a great blog you can follow as well here. In a nutshell, this is a way of thinking about the 80/20 rule. 20% of your products will drive 80% of sales, but what about the other 80% of products? With the offline model, you focus on the 20%, maybe even 15% because of the space constraint. But online, with virtual inventory nearly costless, you can look at trying to approach 100% selection and thus address that last 80% long-tail products. In many markets, the 80% long-tail is big enough to be a multi-billion dollar opportunity.
When discussing selection, most retailers think about the products you are offering, or SKUs which can include a variety of sizes, colors and other attributes (styles). But it's important to note that selection can also mean different conditions (new, used, refurb) or even pricing models (auction, fixed price, negotiate/offer, mark-down). In today's world, consumers want options and that means not only product options, but 'purchasing options' as well.