Candling is important for...
eliminating the clears (day 5) no veins
eliminating quitters-(day 10) dark shadow in a small lump, no veins.
Of course your egg has 2 ends, large air cell end and the tip point end.
day 16 making sure the point is dark. Below is what you don't want to see. By day...
I like an incubator with a turner, then you will be less likely to candle as much as you would if you turn by hand. I just put them in and walk away. I do try and watch to make sure the turners are still working though.
I use a small flashlight, that is powerful. They cost more money than a small one that is not powerful, but worth it. You can find them in a hardware store.