firstly, and arguably most importantly, is that speaker stands bring the speaker to the correct listening height - having the tweeter at (roughly) ear-level when in your listening position. in doing so you will be presented with a proper soundstage and imaging will also greatly increase.
secondly, you will help to isolate the speaker from the rest of your room. this will decrease the vibrations that reach your source and amp (vibrations have a detrimental affect upon such components). in isolating your speaker you will also find that the sound becomes more focused as the energy from the speaker (vibrations) a dissipated/ projected forwards towards the listener rather than downwards into the floor.
hope this helps