Basicaly, DocumentReaderService is a container for all registered DocumentReaders. So, you can register DocumentReader (method addDocumentReader(ComponentPlugin reader)) and fetch DocumentReader by mimeType (method getDocumentReader(String mimeType)).
TikaDocumentReaderServiceImpl extends DocumentReaderService with simple goal - read Tika configuration and lazy register each Tika Parser as TikaDocumentReader.
By default, all Tikas Parsers are not registered in readers <mimetype, DocumentReader> map. When user tries to fetch a DocumentReader by unknown mimetype. Than TikaDocumentReaderService checks tika configuration, and register a new mimetype-DocumentReader pair.