For the sheer convenience for one. Having a whole lot of multimedia can eat up GB and GB of storage, just, like, that. There was a Reddit post where people post some of their largest iOS games. They're easily taking up 3 to 10 GB, apiece.
Somebody else mentioned not having access to wifi. I wouldn't want to have to keep using internet access just to access stuff that could've been easily done locally.
I went with the 64 GB model since to me, it's just going to be a glorified gaming device. However, other people use their iPads as laptop replacements, and it's nice to have the extra space for productivity usages.