For me it's a couple of things - I've had a Nexus 4 since November and while I really like it, the glass back is a pain in the ***, it slides off everything and gets little scratches that aren't really that noticeable, but I know they're there. So I'm not really interested in another phone with a glass back. Also I use my phone a lot for taking pictures and the shutter button is something I really liked about the ZL design. Lastly I like the compact dimensions of the ZL compared to the Z, and I'm tired of "black slabs" and wanted something "different", so the Red model option really appealed to me.
Update: just realized you probably meant functionally from a software/operational standpoint - as far as I know the Z and ZL should work exactly the same, processor and memory-wise they're identical. The ZL does give you the IR blaster as well, but one shouldn't be faster than the other.