It's a good question. To me, the only point is to add crunch to something, but then one must contend with the green-white continuum: if one goes too high, it is nice and green but too rubbery, but, if he goes too low, it is too white and flavorless/bitter (but crunchier).
I don't buy lettuce because it is usually not a good consistency (see continuum above), and it only stays good for a short period of time. Therefore, I only get it at restaurants which can afford to restock frequently and keep only the crispiest of lettuce.
