Well I had trouble finding a 128 GB micro SD where I live so I got two 64s, originally for my blue button Sky 3DS. And some people might not be comfortable with downgrading their 3DS.
I use both. When you have like 26 3DS games installed you take up space pretty quickly.
I like to try different stuff, too. I have two 3DSs, an old with A9LH and a new with EmuNAND.
I also have a Sky3DS+ which I quite like. One nice thing about it is that you can have OoTH and play it as a game. An OoT cart with the hax can't be saved on without losing the hax.
