Metadata Tables?

[mediaformat]

Issue description

Currently, with MySQL there are tables such as config, entities, reader, site which also have their own *_metadata table.
The Entity data is JSON encoded within the contents column, but all of that data is also repeated in the entity_metadata table, with rows sharing the _id of the Entity and the data presented as name, value pairs.

I'm curious as to why it's organized this way?
Was this was a way to handle json before mysql added json methods, or if there is a performance benefit?