1. They keep in the the warmth created by the decomposition process and this speeds up the process. 

2. The amount of water contained in the compost can be controlled more easily. Water the compost bin when the compost becomes dry, cover it with a lid when conditions are wet.

3. The compost is kept moist right up to the edges which speeds up the composting process.

4. Small amounts of compost will decompose far, far quicker in a compost bin.

5. A compost bin keeps the compost enclosed, reducing the danger of wild animals taking cover in an unenclosed compost heap. Bees and wasps sometimes favour compost heaps for nesting. 

6. Finally, they can be used in conjunction with an unenclosed compost heap. Use compost from the bin when finer quality compost is required.
 

COMPOST BINS - DIFFERENT TYPES

There are a hundred or so apparently different compost bins, but most fall into one of the categories below.