My wife and I are new to the game and lost one egg my wife dropped, of course it was fertile and she was so upset I cleaned up the mess in the basement near the incubator( we had 20 eggs and put half in an incubator) Next I went out to Broody MC Buff and looked her eggs over, she was quite mad and if looks could kill. The results were one egg I took and cracked from the incubator and it was not fertile, left all 10 of Brody's eggs because at day 14 I was still unsure. I hope tomorrow when I remove the egg turner at day 18 I can cull anything that looks bad but I fear killing any chicks due to my lack of experience and I think JOSHU and others have a point about letting nature take care of things. Broody was so upset by my day 14 visit she had diarrhea all over the coop and so I had to give her acv water and bread with live yogurt culture. Did the trick but I think my visit and picking up eggs was the cause.
Broody poop can have a life of it's own! Bless your heart for having to clean up runny broody poop - I believe that's a hazmat situation - LOL - toxic clean up on isle 4.

We candled every day with our 1st incubator hatch as it was White Leghorn white eggs and we were studying it as our homeschool project. We tracked development and out of 13 shipped eggs had 6 hatch with 5 living past Day 3. 4 were cockerels and 1 pullet. That candling experience was wonderful because we had a clear picture inside the egg until day 17 when it was just a big blob. lol. The brightness of the flashlight was important as well as the darkness of the room. Even then we still went into lockdown with a few "unknowns" because I wasn't willing to toss anyone who may have been developing.
As far as whether or not to candle - it does take a lot of practice and even then doesn't guarantee a hatch just because you see something developing. Now I tend to err on the side of caution by leaving the eggs unless they are (1) clear (infertile or not developing) or (2) cracked/smelly (possible bacteria - may explode). If the embroyo is not developing properly the broody won't stay on the egg after everyone else hatches and most likely will abandon or kick them out of the nest. If they are developing properly - they will have a better chance of hatching if they are left under the broody's control and not messed with a great deal. Usually she won't abandon an egg with a viable chick in it. Smokey hatched her babies over 3 days. I've had left over eggs from my broody experiences and candling them showed either early quitters or late quitters - but not a chick still developing.
I believe the benefit to candling is mostly at the start and end of the time period - if the eggs aren't developing you can quickly slip new ones under her so she doesn't sit for 21+ days and have no result. And then if nothing has hatched by the 25th day after she started sitting, candling will help you see whether there is movement or life still in the egg. Otherwise, I like to leave the eggs alone to hatch as naturally as possible.