Stories make it possible for an audience to experience places they can't visit in real life, such as fictional worlds and past or future settings. In this sense, the creator acts as a sort of tour guide to the audience, showing them the most interesting and unusual aspects of the setting. Webcartoonists who neglect backgrounds in these kinds of stories tend to focus on the characters instead, but it's a wasted opportunity, especially if they've put a lot of thought and imagination into developing their world or have done a lot of research for it. Characters are important, but an often overlooked dimension of theirs is how the locations they're accustomed to are a part of their personality. There are always a variety of cultures, environments, and social classes to take into account, and those are reflected in a setting's appearance.
2) Making It New Again
Backgrounds are less important in slice-of-life stories than in fantasy or sci-fi because the audience should already have a pretty good idea of what things are like from their own experiences. However, artistic representations of daily life can be engaging since we normally don't consciously recognize environments and objects we're used to seeing. We simply have information overload and can't pay attention to everything. When these mundane places are illustrated, though, the reader can take their time and view the details of each panel, appreciating every object, building, and background character as something the creator felt was important enough to include. In this way, every city, landscape, and room has the potential to be a unique experience for the reader. Creators also have the opportunity to present real-life locations in an interesting way, as it's a lot more convenient to read a comic than to travel everywhere and experience these locations in person.
3) Varied Compositions
There are basically four kinds of views a creator can choose from when conceptualizing a panel: close, medium, wide, and establishing. If a comic has minimalistic backgrounds, it'll generally only use close and medium views, which gets repetitive quickly. Utilizing all four views offers more variety, gives the creator options, makes it clearer what's happening, and allows the creator to present a situation without having to rely on captions or dialogue to explain it.
4) Immersive Storytelling
Rationally, we know that stories are make-believe, but it's fun for an audience to get immersed in the details of a story and care about what happens to the characters. Having backgrounds helps make the fictional world feel "real" and helps readers "trick" themselves that they're looking at objects, people, and places, and not pixels on a screen or ink on a page. When characters are frequently shown against blank or abstract backgrounds, it unintentionally makes it look like they're floating in a weird dimension or void, and that breaks the immersion by reminding readers that it's just a story. Similarly, if a creator tries to show the inside of a character's house by drawing the character standing in the middle of an empty room, it fails to feel "real" because we expect a room to have furniture and objects. It can be distracting, too, if a reader's wondering where the scene's taking place or why it looks empty rather than wondering what's going to happen next in the story.