Welcome to Android Central! Some devices allow you to move a portion of an app to the SD card. If your tablet allows it, you'll see a "Move to SD" button if you select an app in
Settings>Apps. Not all apps support being moved to SD, so if you don't see the button in one app, try a few others.
You can also root the device and use an app like
Link2SD.
Certain apps may have the ability to save data to the SD card, so open an app and go to its menu to see if that option is there.
Otherwise, you are correct--the SD card is really best for storing media files.