Tables with 4 legs rock. Three-legged stools do not. Neither do camera TRI-pods. Three points make a plane. Four points make 2 planes; each competing to be the one attached to the ground. Speakers are often rectangular (4 points). Adding spikes allows the manufacturer to couple the speaker to the ground with 3 instead of 4 points.
Also carpet. They're often called "carpet spikes" because they keep your speaker from "floating" on the sea of shag carpet in your bachelor pad, or the puddle of carpet in your den. Walk up to a speaker sitting on carpet (without spikes) and give it a nudge. Notice how easy it is to wiggle it. Producing music (obviously) causes the speaker to wiggle itself. Spikes help prevent that.