TLDR:

Trailing commas for lists allows easier:

• merging of code
• rearranging of items in the list.

SQL needs more trailing commas so fucking badly.

Commas aren't the only thing that need similar treatment. For example, Typescript allows a leading | for union types, and for union types split across multiple lines, the result is so much easier to format compared to the equivalent code in, say, Haskell.