The file walker is executed at startup (so at setup, after upgrades and after restarts) for the purposes of calculate disk space used. This is what you can disable.
It is also done every few days, when the satellite sends data for garbage collection. You cannot currently disable it. There was some discussion recently to have this instance of the file walker also calculate disk space, but so far with no actions taken.
It affects graphs, but it also affects what the node reports as free disk space to satellites, and so it is sometimes useful to ensure it’s correct.