GU 2001 30lt subtank internals?

[macca1]

Hi out there, Hope you can help me!
Had to clean out the subtank after an algie got in & went through to the main ( blocking filters with black sludge). The algie was killed with additive However still had to remove the top guage assy to access inner tank for cleaning.
My question is WHAT? is the unit that sits in the bottom of the tank there for, it can be removed from the bottom of the tank quite easily , but does'nt seem to connect to anything (round type cyl fitted to some sort of a framework, it sits dead centre & beside the fuel pickup line.

Hope someone can help please ?
Merry Xmas to all out there, Cheers Macca