The custom field request participants has the same functionality like Cc in emails.
In our service desk configuration we have some request types where the customer needs to enter their direct manager (e.g. extend file server permissions, get a license for a commercial software, ...).
Due to the restriction that only agents can comment to the customer (reporter) a default JIRA user picker will not work in this scenario. Every other user than agents also request participants can only write internal comments if the comment is not written via the customer portal.
Also the manager will not see the requests of his team within his customer portal.
Additional add a permission type for this field so that it can be used in issue security levels or notification schemes.