When AV software puts files and various malicious “objects” in “quarantine”, what exactly do they actually do?

I had malware on a computer, which Malwarebytes managed to remove. Or did it? It rather “put it in quarantine”. And it’s remained like that. I see no way to actually delete it.

This appears to not be unique to Malwarebytes at all. It seems to be a standard, established practice in AV software since many years.

But what does this mean? Why not actually delete the malicious files? Why keep them on the system, even if in “quarantine”?