They don't. Many will tell you they do but they were probably told that by a salseman with a powerful amp. Just had a friend over last night and he was blown away by my 2 wpc amp. You listen to the first watt. If it sucks, you don't need 299 more of them.
People have loved to brag about how many watts their system has for decades. I did it in college. Now I know better.
I had a name brand 300 wpc, 120 lb amp and decided I needed new speakers. Put my speakers on the market and a guy came over to listen. He brought his SET amp and we turned it on and I listeded 2 minutes and said, "I"ll honor my offer to sell these to you but if you don't buy they are off the market". He didn't, maybe partially because he realized I wanted to keep them, and the big amp was sold within a week and with a quality SET the speakers shine.
2 nights ago I watched Guardians of the Galaxy with my son at theatre sound levels and my HT system is set up to drive the front speakers with the SET. The house shakes during battle or rocket scenes and the SET could go even louder. You need 1 good watt with sensitive speakers, perhaps 5-50 good watts with less sensitive speakers.
"headroom" is also a marketing term.
I'm sure that there are many about to argue with me: think about it, unless you are listing at ear bleeding levels, your 300 watt amp is putting out .5 to 5 watts and I say that if you have a good 5 watts, then how can the other 295 which are not being used affect the sound?
Jerry