2. Why do some say that the most important erogenous zone is the brain?

Now that even Anna Landers is saying it, you know it must be true.
Sexual response is general based on some stimulus from our five senses, which is processed by the brain. Only when the brain interprets a touch, sound, or other stimulus as erotic does the body respond in a sexual way. Thus, you can say that the brain is the ultimate, or super, erogenous zone.
Actually, the concept of erogenous zone is narrow and outdated, part of our culture’s ongoing attempt to separate sex from the rest of life. The very idea divides each of us into two different bodies, the sexual one the “other” one. From there, it easy to decide that enjoying stimulation outside the sexual zone (for example having your fingers sucked or kneecaps licked) is “perverse.”
Erogenous is a useful word only in very specific instances. Don’t use it to limit the way you think about your sexuality.