I bought my red 3DS XL a few weeks ago and ever since it has never left my side. This is not just for the fact that i love to lay games. There are many reasons to take your 3DS with you everywhere, and I am going to list a few of them.
1. Play Coins
Play coins are currency you receive from walking. The more you walk the more coins you get up to 10 coins a day. Then you can spend those coins on swap note stationary, AR Game items, puzzle pieces or adventures in mii plaza. Its a fun way to make physical activity fun ( I know I have been walking as much as possible to beat Find Mii.
2. Swap Note
This is probably one of my favorite things about the 3DS. Swap Note is a downloadable application in the Nintendo Eshop. Once downloaded you can send messages to any of your other friends who have it. When new games come out or there's a Nintendo direct they send out stationary. Collecting all of them is a fun game within itself.
3. StreetPass
This is one of my most hated and loved features of the 3DS. This feature was originally created for the densely populated streets of Tokyo and Japan. Whenever your 3DS comes within 30 feet of another 3DS they exchange Mii's. Once this Mii is obtained you can use it for puzzle pieces or as an adventurer in Find Mii. In less populated areas it becomes a bit more tricky. I have been trying for three weeks and have only gotten one street pass and it was from the game store demo 3DS. The upside to this nightmare of a achievement is that it seems to bring gamer's from all over together. I know I have met several other 3DS owners in my area in forums and Facebook groups.
4. Mii Plaza
This is where your Mii's are stored. You can use them to beat levels in find Mii, or to fill puzzle pieces on a board. Both of these things can also be done with Play Coins (describe above). So even if you dont get street passes just walk a bit more and you can beat it.
5. 3D Camera
Now at first I thought the 3D feature of this device would end up making me sick, but it turns out to be quite a unique way of capturing a moment. Since I carry mine with me at all times, I find myself pulling this camera out more so then my phone. Although my phone has a better camera, the 3DS can take some really amazing photos. I find myself looking for opportunities to take a photo and send it off to all my Swapnote friends.
6. Music Player
Although I wouldn't put my entire music collection on my 3DS the music player isn't something to be ignored. I find listening to my music in karaoke or 8-bit mode gives me quite a chuckle.