Most of the choices I make in the kitchen are done in the service of function. I brine tough cuts of meat because I know that process makes them tender, and I roast Brussels sprouts to round out their assertive bitter edge. Eggplant is often salted, but not out of habit and certainly not because it's simply what has been done. There's function behind this choice as well.
Salt Eggplant for Your Desired Outcome
The first time I ate an eggplant that was harvested within hours of being cooked, I seriously felt like a small unknown joy has been restored in my life. It was custardy, earthy, and sweet — not a lick of bitterness to bemoan. That, however, didn't prevent it from being incredibly watery and spongey. And it's because of these two reasons I tend to salt eggplant — especially when I want the final product to be firm. As to whether this extra step wastes time, I take advantage of the "meanwhile moment" when cooking, so letting the eggplant sweat doesn't mean other things aren't getting done.
So before eggplant is roasted, it gets salted — or cured, if you will — rinsed, and cooked as directed. And that goes for eggplant steaks and eggplant cubes. Our recipes for roasting eggplant incorporate these steps because it helps draw out the excess moisture that often leads to steaming rather than roasting, a highly likely probability no matter how generously spaced the cubes are on the pan.
Salting also helps draw water out of an eggplant, making room for other delicious flavors. I often salt eggplant, par-cook it, dunk it in a marinade, and finishing cooking it with extremely successful results. Of course, all this might seem excessively fussy to you, but isn't it nice to know what your choices are?
What About Bitterness?
As for the bitterness, this characteristic is being bred out of eggplants. As Sheela mentioned, often times the varieties we eat today are less bitter than those eaten years ago. But beyond any inherent bitterness in the plant, old age and size have a large effect on how unpalatable eggplant can be. Whether left on the plant to grow to enormous size or harvested and left to flounder under fluorescent lights, large, older eggplants are categorically more bitter than their freshly harvested, younger counterparts. And no, this isn't a metaphor for how people are like eggplant. We've got far more pressing conversation about eggplants, metaphor, and symbolism to discuss.
If I end up with one of these eggplants, salting is employed to combat bitterness and remove moisture with one fell swoop. Now tell me, can the microwave do that?