In real life (from what I understand, at least), you can't efficiently store large amounts of electricity, either.
A rechargeable battery converts electricity to potential chemical energy (plus a little bit of loss as heat), which constantly but slowly loses its charge, and when power is needed it converts its stored chemical energy back to electricity (again, lossily). Larger electricity storage systems also convert it to some other form.
By the crafting recipes, IC²'s battery box losslessly converts EU to potential chemical energy, stores it perfectly, then losslessly converts exactly as much as is needed back to EU.
And for the lightbulb example? You turn a lightbulb off by entirely disconnecting its power source, often with a switch located some distance away.
So, rather than building a large structure of pipe loops (which actually seems rather effective storage for powering smaller machines. See how long you can run a refinery or three off a reasonably-sized pipe coil which starts full of power... (and they can look really cool, too)), players want a single magical block that can losslessly convert MJ into a losslessly storable power type, losslessly converts it back, and on top of that, you can pick up, carry it around, and place it anywhere and it retains its charge perfectly. All while outputting exactly as much power as necessary, no more, no less.