If you want to be a good performer...Justification of every action is important.
Perhaps a better word would be motivation.
Bad actors do things without a reason. They walk across the stage, they put their hand on their hip, they stand in one spot etc.
Good actors move with intent and motivation. They cross the stage only to get loser to or further form something. They place their hands in natural positions and gesture to emphasize reaction etc.
If you are doing a trick, and you are fiddling with something, and that fiddling isn't justified (or motivated), it gives you away. The audience may or may not see exactly what you did, but a sharp audience will think "hmm, he just did something there..." and then "hmm, I'm pretty sure he must have switched them when he did that weird thing with his hands"
Someone on these forums mentioned that they want their magic to be absolutely solid and leave no possible solutions. They want things in their act that are perfectly put together. If you want magic like that then justify your actions - every action!
This is why it is great to record yourself and watch for the tells that you have (or anything that might look like a tell).
As far as a justification for using an effect, well, you can do it "just because", but that is a matter of scripting and building a routine, and it makes the difference between an okay show, and an amazing show.