A web site is a strange combination of first impressions and subliminal
communication. If a user can not quickly find an appropriate link to
the information they desire they may leave your site and look else where.
In designing a site it is all too easy to forget that the user will
not have the same knowledge of the site archetecture, as such they may
not be able to navigate with ease.
All sites should be designed with the target audience in mind, but if
usability is of high importance then sites can be tested on a selection
of typical users to check they really are 'user friendly'.