Well, obviously, because you have MacBook Air. There isn't a lot of point comparing iPad apps with full OS X apps- they live in quite different worlds.
I'd further add that if you're a dev type, and expected to remotely fix stuff, that you'd have a laptop anyway. Can't see doing anything more than fixing a typo on the iPad...in which case, I'm sure a SSH app is all that's needed.
Having a local editor for remote files is super handy if your SSH is flaky for whatever reason (say, you fall off LTE into 1x or GPRS land), regardless of the device you're on. Never mind being able to take advantage of the touch screen...
You need to be connected to the network. From their website: "Can I work on my site offline? Not Currently. Diet Coda's focus is editing files on your staging server; if you want to build a whole new website while you're there, that's also cool."