I’ve talked about the why’s of bagel boiling, but didn’t address all the additives that can, and often do, go into the boiling pot. Everything from sugar to malt syrup to baking soda to lye are used as additives, all of which have the effect of creating a darker, crispier crust. Why darker? Because sugars that are deposited on the outside of a bagel as it boils caramelize in the oven, turning the crust nut-brown (the moisture also helps in this regard, since it encourages the action of enzymes). Alkalines like baking soda or lye have the effect of making the crust both brown and crispy, in the first case because they help break starches down to sugars, and in the second because they react with oven CO2 to create a rigid, edible carbonate, the same thing that gives pretzels their crunch.
Me, I add both sugar (about a quarter cup) and baking soda (about a teaspoon) to my boiler, and am quite happy with the results.