Don't worry, I wouldn't do things like this in JSON. Nevertheless, it can be very useful to have comments along with configuration values, for example to explain the actual values (not their purpose) and why they were chosen. That's information you can't add to the code which processes the values.
Ideally, you would use TOML for human-readable configuration and document your JSON API with external documentation instead of sending comments around a bunch. If you need to display the description to the end user though, that would be a valid use case.
json with comments can be parsed by a yaml parser. It's how I write yaml, in fact (yaml is a superset of json. any valid json is valid yaml, but it also supports comments)