Adding onto winners you can increase profits by multiples without increasing risk. This is edge to the max when your exploiting profits. It is not easy & this is a more advanced skill that takes a lot of experience.
Adding on to losers is a bad habit that puts most on the road to ruin. The root cause is typically from using way to much size with no loss exit planned or honored. As a loss spirals down they pile in with hopes of breaking even. Only a matter of time to eat catastrophic losses you can't come back from.
Some, but not many do avg down and still keep their losses small - this is also an advanced skill. Novices trying this are likely to go down in flames.