> a useless one

If you have a (long) list of fields and you want to keep them lexicographically sorted, being able to reorder is quite useful if you rename a field.

Renaming is forbidden though (because JSON and textproto). In Google, it's a documented antipattern to try to make protobuf look "nice" by changing field indices, rearranging fields, etc. — the common ground is that it's better to not do it.