Is your Web site primed for every possible viewer? How would you know if it was? The greatest thing about a usable web site that conforms to current web standards is that anyone and everyone is able to use it.
Ensuring your site is easily usable by your key demographic will ensure that everyone gets where they need to be, and sees what they need to see on your site, without unnecessary effort or excessive clicks. Conforming to current web standards ensures that irrelevant of disability or user restrictions (running IE6 counts as both!) means that no ones left out; this is particularly important on sites that appeal to a completely open demographic, a local government website for example.