Why is this required?

[98.40_for_sure]

When discharging, the products must remain in contact with the electrode. So when recharging, those products become the reactants and thus convert back into products, which then is used as reactants, so on and so forth. So basically the solids/ions produced cannot be allowed to move around and react with different shit because then you can't fully recharge if you don't have all of the shit you made in the first place (however in reality, it will never full recharge anyway due to the products converting to a less active form or solid products falling off the electrodes). If the porous separator was not there, the products would react with each other and energy would be lost in the form of thermal energy etc