Why is there an extra plastic container in my toilet tank?
I believe that you are correct and that the container was added to reduce the amount of water used per flush. As for whether it will affect your fix - it might. If the tank is leaking then unless it is cracked then there are only two places that it could be leaking from - where the inlet connects to the tank or where the tank connects to the bowl. If it's leaking from the inlet valve then the container would have no effect other than giving you less space to work. If it's leaking form the spud washer (the rubber gasket between the tank and bowl) then you may need to remove it to get to the bolts that hold the tank to the bowl (though it appears that the container has indentations that go around the bolt heads so again it may just come down to the only effect being a more cramped space to work).