(Crawling out of some last-minute background updates for a slightly deranged deadline, pausing before finishing review of an operating and maintenance manual...)

There are a few different cases offered for spikes. The one that I'd call most straightforward is stability on carpet - a flat surface or even rubber feet will still allow some motion (rocking, tipping) when resting on a soft surface like carpet, but spikes can penetrate to the floor beneath and let the speaker be supported separately from the carpet. There are other reasons offered (isolation to prevent transmission of vibration), but how effective that isolation actually is becomes a less clear-cut matter - the energy produced by the movement of the speaker drivers still has to go somewhere.