it's not just the fluids. in the ED we always have to think "worse case scenario", and if the patient codes or requires meds that need large bore iv's, then we just do it. also, we may not always know the cause of someone being unconscious. if they didn't know it was dka, they are (again) thinking worst case scenario and making sure there is a good and patent access.
and to add on to what ERjodi said, not only for fluid resus (figure with a 20g you can get 65ml/min as opposed to a 22g @ 35 ml/min) and meds (esp things like D50 which is like toothpaste) that require large bore IV, but also in the ER bc of the unexpected fun that can happen, you have to have a large enough access to run blood thru (16, 18 or 20g) as well as administer IV contrast thru (CT Scans for example require a 18-20g).
when i worked in the ER, almost any and all adult patient, unless totally impossible, got an 18 or 20g from me, traumas i always opted for a 16 or 18. you just never know.