For a few reasons, but the most common is that when you write a file, you 'fwrite' operation doesn't typically immediately write blocks to the disk, instead it gets written to caches that will be written to disk in due course. Changes to the disk, tables with crucial data, etc again are just updated in memory, and written to the disk at a 'sensible' time. It's possible that you aren't actively performing operations to modify the disk, but various previous changes are sitting in memory waiting to be synced. If you kill the power then these changes aren't flushed so you can end up with corruption/filesystem problems. The main reason things work this way is for performance.