When you do a cold start of your tablet, the booting process involves loading parts of the Android operating system and whatever apps that might be preloading into RAM too. As you use your tablet to do different things than some apps and perhaps related OS background services get loaded into RAM, with older unused processes getting killed to make room for the newer processes. When your available RAM indicates there's not a lot of free memory, that's a good indicator things are working as they should be. RAM usage is maintained by the Android memory management and it's almost always more sophisticated and effective than any of us who think we can do a better job at it manually. For most of us, micromanaging how your tablet functions is a good way to learn how it works but don't expect to actually be improving or optimizing its overall performance.
As for clearing the cache as indicated in the title of your query, it might help to first distinguish between the system cache (tied solely to the installed Android operating system) or app caches (tied solely to installed apps). Caches themselves are always just temporary, non-vital, working data so deleting them won't wipe something like files you saved, but they do have a purpose so as an example, when you wipe an app's cache it's going to just rebuild it again later. So delete your app caches as much as you want, but the relatively small amount of internal storage space you're opening up is only a temporary fix. App's need and use their caches. Also, it's important to have an accurate concept of the internal storage in your tablet -- it's divided into several partitions with most of them being dedicated only to the Android operating system and one big one being aside as the user's data partition. The files and directories in those system partitions are restricted from open access to a user by system-level restrictions. All the files and folders in that user data partition however, do not have any such limitations. The system cache is retained within system-protected partitions, your apps and their caches are stored in that user data partition. So there's a clear distinction between the two and they're not interchangeable. You can wipe the system cache in Samsung devices by booting up in its Recovery mode, but even if you do this isn't going to help clear up more space in your user data partition (and again, it is just a temporary matter anyway). The free space in the internal storage media is a matter of how much is available in that user data partition, the system partitions are essentially off limits to a typical user.
(and as an aside, new Android versions in non-Samsung devices have dropped having the system cache retained in its own dedicated partition and is instead just incorporating them into the other system partitions as cache files. So in some Recovery/Fast boot modes the 'wipe cache partition' option is gone now.)
