So much BS in here ^^^^!!!!
Spikes under speakers serve to MASS COUPLE the speaker to the mass of the floor. It helps to change the resonant frequencies of the cabinets by adding the mass of the floor to the mass of the cabinets. Whether or not it makes an audible improvement is highly subjective. But in the case of underbraced cabinets, it can help. It’s as subjective as cable lifters, mono block amps on stands, speaker cables, and component spikes. One definite advantage of mass coupling is using metal wood screws to mount a turntable shelf to the wall studs. It greatly lowers the resonant frequency of the shelf supporting the turntable.