(1) Obviously, to lay the eggs and inseminate them;
(2) To keep the eggs from growing fungus. This is intriguing, as the eggs need constant fanning to keep fungus from forming. Commercial fish breeders often add chemicals to the water to prevent fungus, but I'd prefer to grow the babies naturally. And the most natural method in the world is when the parent remains near enough to the eggs to keep them fans by their side fins. Another natural method is to position an air wand or air stone beneath the eggs to keep them oxygenated.
(3) When the babies hatch, they are not strong enough to swim. So the parents are needed to move them to a freshly cleaned leaf or other vertical point such as a slate or even the tank glass. If they fall off, the parent remains near to gather them into their mouth and gently spit them back onto the spot they cleaned for them.
(4) If there are predators in the tank - other fish of any kind - the parents are needed to vigorously defend their babies. If a filter intake is not properly covered, the parents will also keep their fry far from it until they are strong enough to withstand the intake's current.
But if there are no predators in the tank and no possibility for the fry to be harmed, leaving the parents with them can work against the breeder.
First, the parents will attempt to cull the weaker ones - meaning they eat those they don't believe "stand a chance". An attentive breeder can often spot the weaker ones and give them extra food, as I did a particularly small koi angelfish who is now fully grown and very healthy (see the picture below of Alfreda.)
Second, the parents will become so protective that they teach the young fish to hide when I come near. That doesn't work out so well because when they go to the pet shop to be sold, nobody wants to buy a fish who is afraid of them. Plus, it keeps the fish in a constant state of fear, which is never good.
In the video below, the babies have just become separated from the parents (see yesterday's post). They still have their torpedo shaped bodies. But wait until you see tomorrow's post. You'll be amazed at how much they have grown in a short amount of time.
I especially enjoy seeing them at this age, when they travel only in schools.