I always thought that the actors who were named last, and that had their characters named, were getting some kind of preferential billing without designation them as the star of the show. In the movies, the biggest star gets his/her name above the title (sometimes more than one get this). The "supporting cast" are identified right after the title ("also starring" is a term used), and the designation "with.." indicates a lesser role, or a walk-on by another "A-list" star. TV does it a bit different, and the "and...as..." is a billing just below the main star. I see it as a nod to that actor's success or experiences elsewhere (eg: Alyson Hannigan and the "American Pie" movies and ASH's many years in Britain). The other factor is the importance of the character in the show. You could make a case for Seth Green deserving that "and...as...", but Oz disappeared into the woodwork in some eps, while Giles and later Willow were always given significant screen time or played important roles in the plot of the ep.
So the readers' digest version is that the "and...as..." indicates someone who has increased their visibility either in that show or elsewhere, and is being acknowledged for it.