I candle twice. Once at about 10 days, and again at 18. I do it for functional reasons, to remove bad eggs, to remove clear eggs, to make room for those hatching, and to remove any chance of stink bombs.
But the best reason I can give is I like to see what's going on. Before this, I never hatched eggs in my life, so to see the gradual development is really satisfying for me. I have a personal relationship invested in the chicks right when they are starting to grow feet and feathers, so I feel as if I am closer to them. I also talk softly to them while I am candling, so they can hear my voice through the shell, and maybe, just maybe, that will help with the bonding process too. I raise them as yardbirds, but also as pets, so I like being fully involved in raising them right from the start.
I get thrills in my heart when I see them move, and hear them cheep from within the egg, and that's the fun part about hatching our own eggs at home. I feel like I am directly in contact with mother nature at her most awesome, making new life.
I have 2 batches of eggs going in the 2 incubators now. One batch due around Christmas Eve, or Christmas Day. And another in about 2 weeks. I'm so eggcited.
Sheila