I remember the day I found out that layers had been introduced in Photoshop—it totally revolutionized the program. For the first time I could create complex compositions easily, without having to deal with the less intuitive channels feature or jump through a load of hoops to create separate painting surfaces. Before layers, if you wanted to make alterations to illustrations, or even do something as simple as change a font, you were in for a world of pain. By comparison, I think it is fair to say that layers are absolutely fantastic.
To take full advantage of all layers can do, it helps to learn the options for controlling and altering their effects. With practice, you will find these very useful.