I am a little embarassed asking this, as I was an aircraft electrician at one time, but have since forgotten the answer. When dealing with house electricity, you commonly hear the terms two phase and three phase. Now I understand this to be that the 'waves' are 'out of phase' by a certain degree. But why do we have them? Why multiple phases? Simple so we can tie two together to get 220V for certain appliances? Is it simply a matter of convience that we can bring in more on the same line?