why spikes under speakers???

could you guys educate me about the use or need for spikes under speakers, it seems to me that putting an air pocket under a speaker would be the last thing you want to do, isnt bas about pressure? and if you put a gap of air between speakers and floor arent you losing some of what makes bass work? I am not claiming this to be bad, I simply want to pose my questions about this concept and get educated on why this is a good idea, and when it may not be a good idea...thanks