You have to know this yourself. What do you want to do?
CFW on a 3DS gives you access to great emulators (including the official Virtual Console), allows backing up your game carts, is a great way to learn. Having Checkpoint and JKSM allows backing up saves in decrypted form for use on another console or in case of data loss.
Luma allows funny things like input redirection. Screen blue light filter. Cheats (DON'T DO THAT ONLINE!!)
Access to TWilight Menu++ (and modified TWL_FIRM allowing different scaling filters and even widescreen for DS games).
It's relatively easy now, plus you can play backups (ds game as well), emulate older consoles and use a bunch of great homebrew but the community.
Check it out, its easier and quicker then ever to do it.
Remember to always make a backup of your NAND just in case something happens, although its very safe and iv never had a problem.
Stupid question, but anyways, the 3DS is incredibly restricted, you can't do anything, so that's where CFW comes in. This allows you to remove those restrictions, sort of like jailbreaking for iOS (unc0ver jailbreak was recently released), or rooting for Android. It makes everything better. You can have custom themes to make your 3DS prettier, install games for free, use emulators to play games, use TWiLight Menu++ to play DS games and make your greatest dreams come true, use cheats, and make a backup of your 3DS in case it gets bricked. That is the only reason why I managed to play literally every single Zelda game possible on the 3DS. Once you finished hacking your 3DS, it makes magic happen. So try it, then maybe you will regret asking this question in the first place. Happy Hacking :3