When you put a new pipe into a new fitting and apply solder, how can you be sure that the solder is bonded properly? You can make sure it looks good on the outside, but that's no guarantee the inside looks just as good. There could be a bare spot inside, making it a weak joint.
Would it be better to apply solder to the pipe and fitting separately, then put them together? I don't think many people do it this way though.