Although it's become more common in recent years, a TV show reaching 100 episodes remains a huge deal.
It's the point at which the show can be made available for syndication, and thus offers the networks the chance to make even more money off it, but it also marks a milestone worthy of celebration.
Not all shows take the chance - The Simpsons sent it up by having "I will not celebrate meaningless milestones" as the chalkboard gag, and its younger sibling Futurama missed the mark - but when shows do nail it, it offers up an episode that perfectly encapsulates and celebrates the past, continues the present, and lays groundwork for the future.
Honourable Mention: Brooklyn Nine-Nine - 99
As is befitting of the show, Brooklyn Nine-Nine chose to have their milestone instalment an episode early, making a big song-and-dance out of the 99th episode instead.
While that technically disqualifies it from the list, it's nonetheless a brilliant example of everything the show does well: it has all of its great ensemble together, Die Hard references, Boyle trying to be Rosa's friend, the Jake/Holt dynamic at play, Hitchcock being awful, and a whole lot of heart, even ending with each character saying "NINE NINE!"