Vibration - What are the Main Sources?

Given speakers are a source of vibration, what do folks think they are isolating their speakers from when they try and isolate their them? The other speaker?

Speakers sound best when they are firmly grounded to a solid base i.e. not isolated from their base. Best to spend the effort and $$ isolating other components from the speakers and the vibrations caused by the speakers. 

Transformers vibrate, so it's a good idea to isolate individual components from one another. Stacking components normally produces undesirable results.

@pauly - speakers generate internal vibrations from their drivers moving and push vibrations to the floor vibrating everything.

I’m waiting for you to tell me we use our ears to hear and our eyes to see next. 

That is why you want to isolate them and take the vibrations out of the cabinets.

Nobody “takes” vibrations out of cabinets. Cabinets are designed to resist resonances, and by coupling them to a large mass (read, the floor) their propensity to resonate is further reduced.

Most good speakers either come with spikes or platforms or something to decouple them from the floor. 

Spikes couple speakers to the floor. Take off the spikes and the cabinet will exhibit increased resonance. Coupling to the floor reduces speaker cabinet resonance.

Same is true for amps, phono stages, CD players, DACs, you name it

I’ve never seen a phono stage, CD player or DAC that comes with spikes and meant to go on the floor. All the ones I owned were meant to go onto a properly isolated audio stand.

These components require the opposite of what speakers require. They do not produce sufficient vibration to need coupling to the floor, and benefit from isolation to prevent floor borne resonances getting to them.