Fondant as a cake decorating material is difficult to work with. Before we even get into how demanding it is to sculpt little fondant figures like giraffes and zebras, the issue of covering an entire cake with a rolled out piece of fondant should be covered.
Going over your cake with a piece of fondant creates a very uniform, smooth space and a great backdrop for other cake decor. But if you're too scared to try fondant on a real cake, watch this video to see how to put fondant on a dummy cake. It's good practice.