IMO, spikes are for carpeted floors, pads are for hard (wood, tile, laminet) floors. The real issue is stability if your speakers are off the floor on stands. If on carpet, pads would not be very stable.
On the top plate just something to keep the speaker from easily sliding off if desired. I used cell phone sticky pads that are designed to keep your cell phone stuck to your cars dash. Works great for locking down speakers, and I have them on my table next to my chair to keep my remote control from sliding around.