What is meringue powder?
Meringue powder is a pasteurized egg product - "dried egg whites with stabilizers and anti-clumping agents such as cornstarch".

It is used to stiffen recipes and I think there are two main reasons for using it (as opposed to egg whites):

1) Reduced risk of salmonella

2) Less time/labour/effort used - you don't have to beat the egg whites for a long time to stiffen them

I would guess as well that it would have more consistent results. When making meringues with egg whites you have to be careful not to get any yolk or other fat into the mixture or it won't stiffen properly. There is no real risk of this with meringue powder.