Explanation I use in comments on student papers:
A good paragraph bounds a clear unit of thought, and it provides entrances or exits (transitional logic) from previous units of thought. Avoid fusing all your arguments into overlong paragraphs that should really be separate paragraphs in order to coincide with the natural rhythm of your thought. Distinguishing your thoughts into separate paragraphs will also help clarify where the transitions in thought are and what transitional work you need to do to make the connection to previous or succeeding argument.
