1. You take up space on your SD card by using an emuNAND. emuNAND is also not the recommended setup anymore because you no longer take the risk of losing your custom firmware by updating your system's firmware. emuNAND, for a time, was the recommended way to go because at the time, you would leave your sysNAND at a firmware you could still hack it at while keeping your emuNAND updated for online and being able to use the eShop.
2. Some people use emuNAND for different reasons: to test modifications that might otherwise brick their sysNAND, have a different region NAND alongside their sysNAND, .etc. The list goes on. But now with Luma3DS's region emulation, a different region NAND is no longer needed to play out of region games.
3. You don't need an emuNAND. I'm not sure how many people will walk you through the steps of creating one when it's no longer needed.