I decided I would post this for awareness.

Let us get started. 3DS hacking originated in the early firmware versions of the 3DS 4.0-4.5. This is when gateway released their first flashcart and emunand. Emunand is referred to as the safety net of 3DS hacking (at least by me ). It patches the need for a CIA to be legit (signed) or purchased from the eShop (A.K.A: Piracy) This is not all emunand is used for. It is also used to install rom hacks (such as Pokemon Neo X/Y) Most people using homebrew are only here because they do not have access to a 3DS is firmware that is less than 9.2 or is at 9.2. Homebrew is the loading of unsigned code in the user land so no pircay can be achieved here. (Most people only use emunand for piracy so I will refer to it as piracy because it is faster to type, even though it is not right.) However, some homebrew apps can be installed as .cia files or the games and apps that you download from the eShop.

What CIA files are

Cia files are the virtual copy of a 3ds game or app. Have you downloaded a 3ds game such as super smash bros from the eshop. That was installed as a legit cia because the second you purchased a ticket only created by nintendo was installed to your system allowing the game to be installed. Without it, the game would not install because the 3DS and WiiU both use a system of tickets to tell if a game is legally purchased and not pirated. Even in the process of loading games the system checks for this ticket

Hopefully, now that you know what a cia is the rest will make sense. Cias pop up in the home menu like your system settings. Installing unsigned CIAs (pirated games) is the main usage behind CFW and emunand. As there might be a method to installing unsigned cias to your actual system, there is no way to launch them without firmware patches. Unsigned CIAs can be anything, including homebrew, rom hacks, and others such as system cias like the browser.

Browser CIA usage

There was a person on youtube who installed a browser cia from 9.2 onto 9.8 emunand. This allowed him to do the browser exploit commonly known as load code on the latest firmware

As you can see there are many uses for an old firmware 3ds. The 2DS is the easiest and cheapest to get this sort of firmware on because everybody else charges outrageous prices for this firmware on a 3ds. just remember not to downgrade your system to 4.5 because you will lose access to your 2ds as their firmware only goes as low as 6.0. If you need any help installing emunand google is your friend as I have no experience in the process, only the knowledge.

emuNAND and CFW in depth

Thank you dark_samus3 for the information
Emunand is just a simple copy of your system nand. This allows for risky business that may render your system unusable if you were not on a NAND backup. This is a light explanation. If you wish for A more in depth explanation look at dark_samus3's reply. CFW (again from dark_samus3) is the usage of patches for sysnand. This leaves you with no NAND backups and anything risky could brick your system so be careful! Anything installed via CFW can only be accessed via CFW or emuNAND. (Unless legit and signed)