newbie question - why need to let dough rise twice?

The CO2 gases tend to congregate in their own little pockets rather than circulating throughout the dough.  A degasing and second rise help to redistribute some of these gasses.  It also helps to renew the yeast's activity by providing it contact with new food sources after it has depleted or nearly depleted the sources it relied upon for the initial rise and the second rise.

Taste through chemistry, and then some.

Without going into too much detail the punch down is usually to prevent excessively large pockets of gas forming in your dough. If you did just one uninterrupted proof you would likely end up with some huge air pockets and some tiny ones. The knock-back helps to get rid of the big ones early and keep things even. You want your yeast to act long enough to develop a good flavor while also maintaining a consistant crumb.

Without knowing more about each recipe and method individually I hope that is general enough.