Simply, if you are in party and that durn scout shoots that butterfly you were eying before you get there you still get credit, whereas, if you are going solo, he gets the credit and you have to find another butterfly. It is even more important in the backfield, if you are doing the gargoyles and that other mob, who's name escapes me at the moment, since there are fewer of them. It's all about getting your dailies over as quickly as possible, which is easier to do in a party than in competition.