Separating your work into layers helps to keep everything clearly organised and can also help to prevent mistakes. You can lock layers, hide them, and select everything on them or just select specific objects. You can easily move objects from one layer to another, and you can also move them up or down in the layer 'stack' to change the visual hierarchy of what's on top of what. And, to anyone who reacts with horror at hearing the word 'layers', take heart: they're much easier in InDesign than they are in Photoshop.