Little bugs like living in it. I have nematodes in my gravel. Every now and then one gets out or forced out and the bettas eat them up! I personally prefer the look of something on the bottom to give it a more natural look. I am currently phasing out the gravel in my shrimp tank and they are clinging to the small pods of gravel left (so they know what makes them "look" camoflauged... And I don't like the look of the bottom of the tank glaring back up so it looks like a tank. But that's just me. (fyi, I'll probably fold up a black shirt and put it under the shrimp tank to reduce that effect)
Like bomb said, it does not really super affect the cycle, and in a smaller tank you won't be cycling it anyway bc of the necessary changes.