Fitting the Customers Expectations
A large part of accessibility in web design is ensuring that your website is intuitive and works as the user expects it to. The current generation of web users are experienced internet citizens, and have very well established ideas about how websites and interfaces should work. An easy example of this is the hyperlink. If you see a word underlined and blue, you expect this to link to something related to the word or sentence. Subverting this expectation, for example linking to an advertisement, or not linking at all, can frustrate a user, and confuse them. Given that sales is often an exercise in making potential customers comfortable, this is something you definitely want to avoid.
Not all examples will be as easily avoided as the one above. For example, you should be familiar with how your website is used, and cater to using it effectively in that way without asking your users to change their browsing habits. Humans are creatures of habit, and attempting to design a website which relies on them breaking these habits will quickly lead to user frustration.
A great example of this is in things like online shopping carts. Most internet users have a degree of familiarity with online shopping carts and payment systems. The proliferation of huge online marketplaces such as amazon has led to people forming habits about how such systems work. While it can be useful to innovate in your design, changing too much about what your customers will expect can cause them to misstep. If your design is relying on people to change their deep seated habits in order to use it effectively, it isn’t a good design. Therefore, allowing users to keep a shopping cart while browsing, allowing them to edit it easily at any stage of the checkout, and offering them the ability to save payment information (depending on the type of service you offer) will all help you to offer a service which matches what your customers expect.
An important part of web design is understanding what people expect, how they will act, and giving them tools to make any mistakes less frustrating. If you know users looking for X frequently end up at Y, have a visible and accessible link from Y to X. Understand how people will behave, and help them. While they may not notice it, they will certainly be impacted by it.