Original Patio Plywood Design
This looks like it was part of the original patio
that was built and then converted into a room addition.
The piece of white painted plywood that is nailed to the beam is used to make a connection to the wood post.
A long time ago patios were built without metal connectors and I have never seen one of these patios fall down because they were missing some framing hardware.
You would be shocked at some of the homes, patios or decks I have seen that do not meet today's building requirements and are still standing up in good condition.
Most building departments require framing hardware or metal connectors for a situation where a wood post needs to connect to a supporting beam.
Most building departments would not allow a plywood connection for a situation like this now.