Hdds are a physical drive. If you think of them as being built like having 5 or 6 vinyl records stacked one on top of each other with space for a needle arm between each, you'd be close. They'll last a good long while with normal use, but once a record is scratched, it's scratched, there's no repairing it, all you can do is skip over that section of the song. Get a big enough scratch and it'll bounce the needle hard enough to send it flying in the wrong direction, creating more scratches, up until the point the record is trashed, useless, and the once good song is now just squeals and corrupted notes.
WD was smart enough to color differentiate their models. Greens are for slower, long term little use larger files. Reds are built for NAS, Purple are built for 24/7 power on constant low usage, Blues are general purpose, Black's are high performance (gaming), Golds are the Black version of Purple. Each is built with slightly different strengths in mind. For a standard usage pc, Blue or Black is recommended, that's where their strengths are built for, higher speed, data storage, intermediate read/writes usage.