It can be a contractual thing, particularly in ensembles, to set a particular actor off from the others as a way of drawing attention to them and giving them an air of prominence in the credits.

Examples: Thomas Gibson and Paget Brewster both, at different points, were given "...and" treatment during the run of Criminal Minds to establish their status as the unofficial leads. If I remember correctly, Tom Selleck gets the same treatment on Blue Bloods.

For special guest stars, you're thinking of "...with" billing.