Because it follows the general principle of making it easier to change [0]. Fewer edits, cleaner diffs.

Useless from the perspective of a wire format, but nice for things like config files, which seem to be the use cases json5 is targeting.

[0] Bullet pt 3: https://betterprogramming.pub/5-essential-takeaways-from-the...

When manually modifying JSON, it really helps avoid accidentally missing a comma when you add or reorder elements.

JSON isn't English. The comma doesn't mean what it does in English, so I don't see why a period would be appropriate either.

Makes copy-pasting and reordering lines easier, which is quite handy.

Simplifies JSON-generation code, typically saving a conditional.

recursing issue since forever, "separator" v.s. "terminator" both can work. For data rows, I prefer termination as makes missing columns explicit. But being optional here, it means nothing.

I guess it facilitates low effort scripting that doesn't have to treat the last element differently.

Just noticed today that the Python formatter "black" does this, I really disagree. Of course I realise that by noticing its changes and arguing about them I am missing the whole point of using it.