Even though you can't see an underlayment or a subfloor, it's one of the most important elements of flooring installation. An improperly installed underlayment can cause insulation problems, and can even cause the floor installed over it to buckle, warp, and break.
There are many types of improper flooring installations, and the most common one is when the underlayment is inadequately prepared for the floor that will be installed upon it. An underlayment needs to be free of dirt, debris, jutting nails, and anything that would cause the surface to be uneven. Hardwood floors, especially, are sensitive to an improperly installed subfloor, and will quickly buckle & warp if installed on an improper underlayment. In addition, if you install an underlayment in an improper environment -- in an environment that's too moist, or too dry -- it can cause problems down the line.