When you are farming fragments for a 3* Ninja you might don’t want to recruit him asap when you collected those 80 fragments. The reason behind this is that you can always be lucky and draw the character with seal scrolls or from survival trial. If you do not own the Ninja you get the full amount of 80 fragments leaving you with the 80 fragments you farmed, just 20 away from 4*.
But if you recruit the ninja the moment you get enough fragments and you get lucky a day later the draw will transform to 20 fragments, which means you will still need to farm another 80 to make them 4*. So before you use the hard farmed fragments you might want to think yourself, if you really want to use the ninja asap or if you can wait and maybe benefit that way in the long run.