Putting rocks or gravel, or broken pottery or popcorn styrofoam in the bottom of pots is needed to assist with drainage. Especially if the pot does not have any drainage holes in it. Very few plants like to sit in standing water, unless they are bog plants. Because the plants are not in the ground, where water can just seep away if there is too much, the rock keeps a bit of space open for the water to seep to if the soil is too wet.
To actually make this work more efficiently, you might try putting a coffee filter, old nylon stocking or something pourous between the drainage material and the soil. This prevents all the soil washing into the rock and filling up all your drainage so there is no where for the water to go.
Of course ideally, your pot should have drainage holes, and a tray underneath, in which case the drainage material is not as necessary, although I use it anyway.
I use a combination of all three, rock, gravel and pottery in all of my container planting, indoors or out.