Why do some DACs have transformers?

...and some don't?

I have both and don't understand why.

I'd think all do as they have to transform the wall juice to dc. Some are outboard that's all and I'm sure it's to keep noise down. I'm no ee though so maybe I'm wrong.
Some DAC's have inboard transformers as they have inboard power supplies. Others have wall warts as their transformers. Which do you think has better power regulation?
I had a Bitwise DAC with three transformers. It was a sweet sounding unit. Why the heck did I sell it?
Of course, of course! What was I thinking. They all have to have a transformer. I didn't think about the wallwort!

Thanks for the replies