Online scanners not realy online???

Hi, I wonder how online scanners work. I first thought they where scanning my computer from the internet, but of course that's not possible. Would take days to upload my hard drives... I discovered that most of the online scanners are installing around 100 mb or more on my hard drive. Mostly in the hidden files like documents/user/local/temp and so on. So a online scan is just running on my own computer, and updates are installed on my own computer. So what's the point of using a online scanner and not just install a on-demand scanner like MBAM on my system. I also noticed that some online scanners keep running as a procces in the background. Somebody can explain to me how online scanners work? Thanks a lot!