Use it to store your media. You can set your camera to save to SD card. If you have spotify premium, you can set it to save downloaded songs to SD card.
Some apps that have additional data also save to SD card. Others you'll have to move yourself. But in essence it's just a way to offload extra data to an additional memory.
There are options to use it as an extension of internal memory, but I don't bother with that myself.