What are the benefits of one?
Yes, There are many foods, ie Chocolate, Cheese, Sauces, that cannot reach a temperature of over 212 degree which is the boiling point of water. Thus a double boiler enables the contents of the top part of the pan to NOT EXCEED 212 degrees. The above foods will be ruined if you do NOT use a double boiler to melt or make them
they are nice for steaming and melting things with out burning but you don't actually have to buy one. i usually will use a pot and boil the water like normal and put a heat safe bowl on top of it or another pot but IT WILL BE HOT. i never actually went out and bought one or anything
To heat/cook sauces or dishes in which very high heat could spoil, or just holding them hot for very long time without overcooking. We have 2 big ones at work and I use them to make the gravy and custard ahead of serving time while I cook other things.
Unless you're a pastry chef or a chocolate maker, I'd say no. These days, most people put a metal mixing bowl over a pot of simmering water to get the same results as a double boiler. That is fine for Hollandaise sauce, chocolate dishes, etc.
You don't need a formal one, a make-shift one works just as well. But yes, if you melt chocolate or make delicate egg-butter sauces that can break if they get too hot, you need a double-boiler. It's gentle heat using steam that is even all over, no hot spots like a pot can give you for delicate foods. I put a pyrex glass bowl over a saucepan of simmering water to make a double-boiler. I'd never spend the $$ to buy one that fit inside a saucepan. Just one more thing to have to store.
