The X Plugin introduces another connection protocol (listening on port 33060) as an alternative to the mysql connections typical of port 3306. Having first discovered the existence of mysql's port 33060 using network monitoring tools, that much seemed practically self evident. So, it can be slightly frustrating to start reading documents that explain port 33060, per se, because they explain "X Plugin" as if you knew you wanted it, already. These don't explain what features would require the second protocol (i.e. "why?"). Fortunately, that is elsewhere in Mysql docs, on the page for "Document Store."

Excerpts from that page:

To use MySQL as a document store, you use the following server features:

X Plugin enables MySQL Server to communicate with clients using X Protocol, which is a prerequisite for using MySQL as a document store. X Plugin is enabled by default in MySQL Server as of MySQL 9.3. [...]

X Protocol supports both CRUD and SQL operations, authentication via SASL, allows streaming (pipelining) of commands and is extensible on the protocol and the message layer. [...]

Clients [...] using X Protocol can use X DevAPI to develop applications. X DevAPI offers a modern programming interface [...]