From the Comox Valley Echo, Tuesday 2 July 2013, Ask a Pro
Question: Does my composter need to sit on earth or can I put it on the patio? 
Answer: Good composters sit on earth or lawn to take advantage of the diverse microbes in the soil underneath. Worms and macro-organisms will be attracted to the feast. Water can transfer and keep the moisture content more stable. In a warm moist environment, bacteria can double about every 20 minutes. So one cell could multiply to over a billion in 10 hours. Talk about compound interest! They will make fast work of your organic scraps and leave a smorgasbord for the worms. And all that rich nutritive material added to your garden will feed your plants to keep them healthy and lush. One way to take advantage of the soil organisms is to place your composter right in a garden. The soil underneath will become luxurious. There is so much life in the soil it is you might as well take advantage of it.