Buffy the Vampire Slayer has a Multiverse too. Let's face it, Multiverses are all the rage nowadays. There was a time when they were largely restricted to a handful of sci-fi shows and comic books, but lately they've been integrated into some of the biggest superhero and sci-fi franchises of all. The next few years will see both Marvel and DC explore the Multiverse on the big screen, although they look set to take a subtly different approach.
Technically Buffy the Vampire Slayer has always had other dimensions. The Slayer is generally the one who defends the Earth from monstrous extradimensional beings, and the Hellmouth is an area that allows Hell Dimensions to invade the planet. But the classic sci-fi trope of alternate universes has never been explicitly explored in Buffy the Vampire Slayer, although it's possible Anya created a branch reality in "The Witch," given Willow was able to accidentally draw her vampire self out of that timeline. Still, that's as far as the franchise has gone - until now.
The current range of Buffy the Vampire Slayer comics are essentially a Buffy reboot, a relaunch in which the titular hero's adventures are set in the present day. Angel & Spike #16 ends with Angel's crew transported to a very familiar hotel from the TV series, and they are told they have traveled through an interdimensional rift. Alternate versions of Angel and Spike enter, welcoming them to the Multiverse.
The "Hellmouth" comic book event ended by escalating this even further, with Giles and Anya conspiring to understand the nature of Multiversal evil. Anya has been compiling maps of the Multiverse, identifying "dimensions upon dimensions upon dimensions upon dimensions," with the Hellmouths serving as bridges between them all. Giles is horrified, wondering how the Slayers can possibly protect so many versions of Earth, but Anya tells him he's thinking too small. Hellmouth #5 closes by revealing just why Giles is wrong - by introducing another dimension, one in which Willow is Sunnydale's Slayer and Xander and Buffy have been friends since childhood.
The introduction of the Multiverse is frankly inspired, because it means the current comics can coexist quite easily with the classic TV series; they are simply visions of different worlds. This also clears the way for any potential Buffy TV reboot, such as the one first teased in July 2018. The truth is that Multiversal plots and alternate Earths are often introduced for purely functional reasons, because of the opportunities they offer; that is certainly the case in Buffy the Vampire Slayer.