Audacity performs peak normalization, but has a fixed setting for the highest peak, for some reason: -3 dBFS. When normalizing, most people want to increase the highest peak to just below 0 dBFS (like -0.1 dBFS or some such). Using Amplify allows one to specify peaks higher than -3 dBFS (like, -0.1 dBFS). I'd just use Amplify.