Need? You don't need to - normally. The kernel will handle it. On the other hand, if you're low on resources and want to free up resources manually you can do so.
For the most part, just let the kernel do it. The commands exist for a reason, but you normally won't need to worry about it.
Let's say you have 4 GB of RAM and you're using 2.5 with another 1.5 claimed as cached. You want to open GIMP to work on a large project. You can manually drop the cache(s) before hand, rather than starting the application and waiting for it.
Or, if you've just closed a resource intensive application and won't be opening it again any time soon, you can clean the cache(s) manually instead of opening an application in the future and waiting for the kernel to deal with it on its own.
So, does it have a use? Sure... But, for the most part, I'd just let the kernel deal with all that. At the same time, I have gobs of resources at my disposal.