Finally, I surf the whole site to be sure the flow is fluid and graceful. I frequently make changes to improve the look and surfability of the site. Part of the functionality or function of the site is to dispense pertinent information in an easy to find manner. Now you see why the opening statement of the paragraph makes sense and why I discussed these functions first. The last and possibly most important part of the functionality of the site is meeting the marketing and interactivity elements and goals of the site.
Lets discuss interactivity first as it leads to some of the other goals.
Interactivity is possibly your best marketing tool on any site. I say this because the only way a purchase or request can be made is by interacting with the site usually by means of a form. Just a little less annoying then a blink tag. To my way of thinking, though cool, these are just added fluff and another way of limiting the same content going to all users. I could go on and on about download time, that some are browser and platform specific, but who really cares.