Acoustic benefit. A carpet between the speakers and your ears absorbs the first reflection point from the floor. Similar to the sidewall first reflection point.
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I think that what you are asking about is a little square of carpet under the speakers, and not the entire floor. Well, it does protect the floor, and also there is a possiblity that if you use discs under spikes on bare floor, the will be some buzzing, and perhaps movements at louder volumes. Carpet prevents this.