XHTML Strict is the "pure" form of XHTML, which is HTML re-formulated in XML. It contains no presentational tags, and is instead aimed to be used in a purely structural manner, using CSS for the styling of the content. This means no <font> tags, no align="" attributes and no <i> tags. All this in addition to following the normal XHTML and XML rules - all in lowercase, no improper nesting of tags, etc. XHTML Strict in particular makes pages easier to read by those with disabilities, since there are no style elements present on the page, and emphasis on words is carried by the <strong> and <em> tags. Users can then use, say, a screenreader and the emphasis will be carried over. Images become less of a problem to those with them turned off as they are required to have alt="short description" attributes with the option of a link to a longer description (longdesc="").
In short it makes for very clean and clear markup that can be easily understood. One of the problems is that browsers support sloppy HTML coding, and so hundreds of amateur authors make their fan sites and homepages in that way, barely knowing about HTML Standards themselves, let alone XHTML.