The goal of an speaker stand is to hold the speaker cabinet perfectly still relative to the free moving speaker voicecoil/cone assembly, which ensures that most of the electrical energy (musical signal) imported into the voicecoil is converted into acoustic energy (some will always be converted to heat). If the speaker cabinet is free to move, some of the electrical energy will go into moving the cabinet instead of the cone. When this happens, the musical signal becomes distorted since portions of it are waggling the cabinet around instead of driving the loudspeaker cone.
Spikes and Blutack work in slightly different ways to achieve the same goal. Bluetack couples the speaker to the mass of the stand, making it too heavy for the speaker motor assembly (voicecoil) to move for all signal frequencies and amplitudes, there is less resistance in moving the speaker cone + air than there is in moving the cabinet + stand. This is why adding mass to a speaker cabinet and/or stand improves the quality of the acoustic signal we hear.
Spikes on the other hand effectively reduce the contact area of the speaker relative to its mass, making it far more difficult to move. To use a real life example, imagine you are pushing a 100kg box across a shiny flat floor. Hard to move. Now place the same box on 4 small nails sticking up from the same floor. Impossible to move. Why? Because the contact area has been reduced from say 0,25m2 to 0,00000025m2. In the first instance, the box is excerting a force of 4.000N/m2 onto its contact area. On the nails, the box in excerting a force of 4.000.000.000N/m2 onto its contact area. Again, from the music signal’s point of view, the effect is to make the cabinet appear to be a much heavier load to move compared to the speaker cone + air.
In summary, speaker stands with spikes or mastic prevent the cabinet moving relative to its surroundings and relative to the speaker cone.
Spikes work well in almost all cases and are probably the best technical choice, especially for rather light weight speakers and stands. This is because the effective mass gain is a multiple of the cabinet weight. Spikes have the obvious disadvantage of marring the finish of speaker cabinets and are increasingly difficult to handle as speaker weight increases. In that case solution is to put a metal plate beneath the spike to prevent any damage.
Blutac works best when used with heavier speakers. The effective mass gain is much lower, equal to the weight of the stand but is enhanced when the speaker stands are themselves spiked to the floor. Prudently adding mass to speakers and stands will enhance the effect of both.