It allows each line to be completely independent of it’s neighbors; you can comment and/or add lines without needing to touch neighboring lines. Also, it makes it visually easy to spot missing commas. Give it a try sometime, it’s actually quite nice.

Wouldn't this also be solved by allowing trailing commas?

But they are not independent: first line doesn’t have one.

Also makes nice file diffs!

Wouldn't the same thing occur if it wasn't there at all?

Having prefixed commas is a rather common style in the Haskell community, because it ends up nicely matching open/closing brackets/braces and lining things up.

Since the author of Dhall comes from the Haskell community, he's kept this style.

Author here: This is correct. I'm just borrowing a Haskell convention. Also, I like this convention because it leads to vertical alignment of commas.