Over the years, Adult Swim has been a lot of things to a lot of people, but it all started as an unbudgeted hail mary project centered around repurposing 30-year-old animation cells.
Back in 1992 Cartoon Network wasn't much more than a rerun channel, thanks to Ted Turner acquiring the MGM cartoon libraries. Their first foray into original programming was headed up by Mike Lazzo, and turned the original cells from the 1966 cartoon into a surreal talk show. That show was Space Ghost: Coast to Coast and would go on to influence contemporary programs like The Eric Andre Show. (Lazzo would later become the senior executive VP of Adult Swim.)
Since those early days, as YouTuber kaptainkristian explains, Adult Swim has become a haven for risky and functionally unrelated shows. Some of the early programming -- like Sealab 2021 and Harvey Birdman, Attorney At Law -- used the same technique as Space Ghost, while others like Home Movies were wholly original.
Adult Swim also became a sort of isle of misfit toys. Rick & Morty and Children's Hospital both started online until they were scooped up by the network. It helped to revive Family Guy and Futurama after they were cancelled. And it brought adult-oriented anime to a mass audience after Toonami got the axe.
The diversity of content -- not to mention the breadth of music and art featured in Adult Swim's on-air promos -- shouldn't make any sense. But as a single unit, swathed in a feeling of late-night transgression, Adult Swim has become the voice of a toon-loving generation.