The reason doug says that is so you can have the gain up, say 2 o'clock, during louder shows rather than rolling it way back to the AM range.
Its a -20db pad, so the decision to engage it or not depends on how loud you think it will be.
I find that for most of the stuff I record I won't have enough gain with the pad on, but YMMV.. generally speaking the gauge I use is if I have to yell over the music to communicate, the pad is needed..