What might membranes within organelles do?

Membranes within organelles may be involved in compartmentalisation. This may be to create a specific condition needed for a cellular process. This may be an acidic pH within a lysosome to break down cellular components.

However, within mitochondria the double membrane, Cristae, increases the surface area for the reactants of respiration.

Organelle membranes serve to separate the functions occurring within each organelle from the cell. If organelles didn't have membranes, none of the functions would be able to occur, because all of the enzymes and substrates would 'mix'.