Take a picture, they say. It'll last longer. But fans weren't always able to do that. Only in the last five years or so, with the rise of digital cameras and smartphones, has photography even been an option.
Even now, some artists insist on no photography at their shows, and a few venues still ask customers to leave even purse-size digital cameras in their car, but it happens less and less. There are just too many of them -- and people need their phones, like for emergencies and stuff -- that many places have decided they just don't need the hassle.
So you go to see your favorite band and you want something to remember them by besides a T-shirt, so you bring a digital camera or take some video on your iPhone. And you're far from alone. But why? Sometimes all those flashes and LED screens all at once make a cool visual effect, but most of the time, to those of us who are trying to just watch the band (or, perhaps, take notes for a review), it's just annoying.