Long runtime, I guess. Maybe the circuit is recycled from a dual fuel 1AA/14500 light. Since they made the housing large enough to hold three AA's, having it accept a D as an alternative is a nice touch. Countycomm has a 1D light that looks kind of nice but costs $65 so I haven't been too interested. For non geeks though, D cells are a simple way to have quite a lot of energy on hand. It unfortunately looks like the AD01 is $70+, oh well. Wake me up if you see a light in this format at $20 or less, heh.
Countycomm version: https://countycomm.com/products/1d-flashlight-by-maratac
Added: oops this post is misplaced. It was supposed to reply to Reader9's post about the Lumintop LD01. Anyway I do like the 3AA/1D format in principle. It just misses its best audience if the light is expensive.