Why wouldn't the hard drives be turned off at the same time the laptop
goes to sleep?
They are, you're correct. What you're getting hung up on, though, is that the hard drives can be turned off BEFORE the computer goes to sleep. They don't have to be constantly spinning for the machine to operate. That's the reason for a separate timer; if you want them to spin down but still have the machine running. (These days, there aren't a lot of scenarios where you can go for extended periods of time without accessing a drive at all, but they do exist.)
My personal recommendation is to set your drives to never spin down unless the machine is asleep (so change that setting to 0 minutes, which registers as 'Never'), but then my personal recommendation is also to never sleep your machine so I may perhaps be biased. Modern computers don't really consume that much less power at sleep than idle - as long as the MONITOR(s) is(are) sleeping you're probably fine. (Obviously if you run a laptop on battery a lot, you may want to ignore this advice for battery operation.)
EDIT: Read the comments for a bit more discussion on this, it seems I could have worded this better.