Sourdough Bagel questions

I just made a close to a dozen bagels. This time they look a lot better than my first time. I have a few questions for you experts out there. 

1, what is the purpose of placing the bagels in the fridge overnight? They looked like I could have just went to boil or maybe in the fridge for 2 hours to firm them up. Is over night necessary?

2, when most of you make sourdough bread or bagels do you add yeast to the recipe? I thought the whole idea of using sourdough starter was to bypass the use of stor bought yeast?

thanks in advance,

steve