It would be easier if you pick up the parts that are not clear. Sam answered really good, we use normalisation to make everything sounds as loud as possible. It has more than one benefit, but it depends of course from many views, how much these benefits are important to us. For instance, if you want to listen to a playlist of a songs, you don't need to mess with the volume bar on each of them, just set it once for optimal, every other song will sound with "right" volume. If you normalise a fairly low volume sample, messing with it after will cause more precise sounding. Lowering a volume by any percent is not a problem at any time, but if you need more stronger volume than your current sample totally has, it's usually a trouble (Jojo stated this problem 2). I could think of some more examples if needed, so what are the questions about the topic, that are still not totally clear.