Youllin3 explained it perfectly. I'll add, I love the new Epcot show because it is truly at FW show, so you can see 98% of it from anywhere. I say 98% because they do some nifty kite/fw thingys that don't fly as high as the FWs and if you are sitting on a bench back away from the lake during a time booths are set up for a festival, chances are you aren't going to see every kite or every country light up. Some of the kites do fly pretty high, I watched from the bridge at BW one night and was able to see some of them over the trees even.
I'm not as huge a fan of new MK show as most people are. I used to be able to get a spot on the bridge right under where Tink flew and see the whole show. Now that it is heavy projection based, you can still see the FWs but not the projections. The two times I've been a good little Disney fan and gotten a good spot early enough to see the castle either a very tall very wide man came and stood in front of me at the very last minute before the show or a father decided to plop kiddo on his shoulder and dance around so there was no way I could lean without him in my way. Same thing happened at HS. One time I was actually right behind a row of electric wheelchairs, we were all very happy with our spots, last minute two men came and stood right in front of the row so the poor wheelchair people had to kind of stand up lean in their chairs, thus blocking me. I was on the outside where they put the walkways so I could kind of lean out and see part of it. I'm height challenged.