over time, the sand can clump and harden..if it's hard, it can't filter. and when i took out my sand it had all kinds of guck in it..almost like cob webby vegetation material from filtering...now, there are a few people here anyway who remove their sand and actually clean it every once in awhile. my sand was at least 5 years old when i replaced and had a bit of clumping..and i had never taken it out..it may well have been in longer...there's a chemical cleaner you can put into your sand to supposedly "condition it' and clean it up. i ended up getting away from sand and going to zeobrite..better filtering than sand..supposedly will clean out better than sand when flushing the filter. i did find that my backwashing didn't seem to have as long term effect without needing a reflushing as it had 5 years before.that and an upgrade to most of my pool equipment prompted me to replace the sand.