With bracket attached to shock -
Pros
Dont have to worry about getting the shock angle right to slide shock on
Only have to slide 1 bolt in to keep the shock compressed
Cons
Getting the one bolt in can be tough, and getting it through both sides of the chassis even tougher
If re-using original brackets, you have to remove 6 bolts
Without
Pros
Far less work
Cons
Have to compress shock without much surface area(if a high pressure shock, can be a pita without a jack/lift)
Angle has to be right to slide shock on