Double boilers, also (perhaps originally) known as a “bain-marie” are used to heat more delicate foods such as chocolate and some sauces that would break down, molecular-wise, under direct heat. Because it heats slowly, and because the heat source is the steam underneath the top pan, a double boiler allows for an even distribution of the heat, thus lessening the possibility of scorching or cracking that can happen with many heat-sensitive ingredients when heated directly on the stovetop.
