its just there to block the gap in the PSU shroud so it looks flat if you don't use that gap for a fan or Water Reservoir for water cooling. it also helps with airflow as if you only use 2 fans in the upper area, it won't let air go into PSU area, and points it towards GPU.
With fans in the bottom area you have to remove it to install them. removing it also allows air to flow above shroud
It isn't essential, more for appearance and airflow management.
since i only have 2 fans in upper area, I left it in.
edit noticed you only had 2 fans, thought you had 3.