The userID (username, not normal for and lastname) is the only identifier to check who is the owner of a series or mediapackage.
It is often necessary to check against external programs, maybe more necessary with the upcoming external API.
This requires that the metadata is available via .json requests.
An exampple: you want to get all videos recorded by user "peter petersen". If your institution has more than two personen with this name you can only seperate them by their userid. Especially relevant when you check between an LMS and Opencast. You would now be able to check via .json which userid has recorded which video.