Ideally it always better to keep the fry with the male and are it's believed that they can get beneficial bacteria from the faeces of the male.
Sometimes members have BN's spawn in a community tank or where there are tankmates that could eat very young fry or possibly injure them, the fry can be kept in fry savers, in the absence of another tank, untill they are less vulnerable.
I think the main reason for using fry savers is that there are occasions when viable eggs can be kicked out or dislodged from the cave. Young males sometimes take a couple of spawns before they get into the swing of caring for the eggs. Often placing the eggs in a fry saver with some aeration will result in the eggs hatching.