Την κασάρει για να την έχει σε ώρα ανάγκης. Η μνήμη δεν χρησιμοποιείται πρακτικά, αν και πρέπει να πω πως είναι πλέον ψιλοάχρηστη η γραμμή αυτή στο conky. :P
The reason Linux uses so much memory for disk cache is because the RAM is wasted if it isn't used. Keeping the cache means that if something needs the same data again, there's a good chance it will still be in the cache in memory. Fetching the information from there is around 1,000 times quicker than getting it from the hard disk. If it's not found in the cache, the hard disk needs to be read anyway, but in that case nothing has been lost in time.
