While watching the May 14 finale of Fox’s “New Girl,” you may spot a familiar face among the guests at Cece’s wedding. That face belongs to the enormously famous — and often contentious — singer, Taylor Swift. Despite hosting “Saturday Night Live” and having a small part in Garry Marshall’s “Valentine’s Day,” Swift is not an actress. Her celebrity status and all the buzz surrounding her personal life was what led to the casting.

“New Girl” is hardly the first television show to bring on a celebrity seemingly for no reason other than to boost ratings. In 2008, a then-declining “How I Met Your Mother” marketed an episode around the appearance of Britney Spears. The episode drew in many new viewers, and five years later, “How I Met Your Mother” is still on the air. Many credited Spears’ appearance for the turnaround.

“Glee” did something similar last fall. When trying a drastically different format for its fourth season, the show brought on Kate Hudson and Sarah Jessica Parker to tempt viewers into taking an interest in the new, split-storyline narrative.

Another vehicle for celebrity actors is the Emmy Award for Outstanding Guest Performance. Sometimes it’s awarded to big-screen stars who have recurring roles, such as John Lithgow on “Dexter,” but other times, the award is given to someone who has been in Oscar-nominated films and only makes a brief cameo. For example, last year’s winner was Kathy Bates, who won for her role as Charlie Harper’s ghost on “Two and a Half Men.” The award offers lesser television shows the chance to win something small.

The stars also have a lot to gain from guest appearances. In 2008, Spears was waning as much as “How I Met Your Mother,” and her presence on the show did great things for her reputation. Other celebrities like James Franco and even Al Gore have played exaggerated versions of themselves on “30 Rock.” A willingness to make fun of oneself can have a drastic impact on a celebrity’s public image.

Television has also been a stepping-stone for some celebrities in becoming famous with a larger audience. “Glee” is notable for taking performers previously known mainly to theater fans — Lea Michele, Kristin Chenoweth and Jonathan Groff, for example — and introducing them to the public. Chenoweth’s season one story arc introduced many casual TV viewers to the Broadway performer.

While the television show gets a ratings boost and the celebrity guest star gets publicity, it’s ultimately the audience who wins. Rarely do celebrity guest stars turn people off. Even Taylor Swift, who has her fair share of haters, isn’t enough to drive off a rabid “New Girl” fan. The random celebrity appearance is an exciting way for shows to mix up their predictable formula and create an altogether more interesting, fun-to-watch product.

A version of this article appeared in the Thursday, April 11 print edition. Valerie Nelson is a staff writer. Email her at [email protected].