Unlike a sack of potatoes, a paragraph needs a shape. A paragraph is not a bag of rice. It's not a tub of popcorn.
A paragraph is a collection of sentences that move the reader to follow along with the writer while he/she explores an idea that is introduced in the topic sentence.
So often, people assume that we have outgrown being mindful of paragraph shapes--have somehow decided that it is too much trouble to begin a paragraph with a topic sentence.
But when they are reading, people need a topic sentence at the top of a paragraph. They need the bridge that it makes by connecting the reader to the idea that the writer hopes you will be interested in enough to follow.
Unfortunately the shape of the paragraph has become a loose and pitiful thing. It is more formless than it has ever been, largely because people forget that they are writing in different environments.
Different environments, like different occasions, dictate the nature of your attire; and when you are conducting business or communicating professionally, the attire is more structured.
Paragraphs should reflect that structure.
When they don't, they are really only the inside of the message you mean. Look at your last casually written paragraph and ask yourself: What's there? What's missing?
If the paragraph has exactly the information in it that you wanted to send, then you have written the middle of your message but that doesn't mean that you reader will know how to process it, because you did not supply a bridge--an introductory sentence that lassoed your reader and held onto him/her until you loosened that loop--that paragraph--by writing a concluding sentence that primed your reader to draw the conclusions that you want him/her to make.
See? Controlling the shape of the paragraph is about steering your readers to where you want their attention to go and then releases them when you have reached that safe place where agreement can be established.
The next time you have a message to share or information to report in a workplace situation, ask yourself: am I stuffing a sack with potatoes or writing a meaningful paragraph? If the answer is the latter, then get yourself a lasso--a topic sentence--and catch a reader before you dump the message on him/her.