Is this plastic container in my toilet tank necessary?
No, the plastic sub tanks are not necessary. They are an after market modification to an older toilet design used to supposedly conserve water usage. 

From looking at the picture of the inside of your toilet tank I would think it would not be a bad job at all to temporarily de-mount the tank so as to fully remove the baffle unit. You could also replace that antique float ball filling valve with a newer style that has the float integrated into the fill valve itself. This would leave room inside the tank if you wanted to try the idea of placing one or two large plastic soda bottles filled with water into the tank as a displacement to save on amount of water per flush.