Today I was helping someone who was having a hard time with reading. I suggested to her that she read the first sentence of each paragraph to find out what that paragraph was about. When she was done, she had a good idea about the overall direction of the passage, as well as what each paragraph covered. We then discussed how the first sentence of each paragraph, also known as the topic sentence, is basically an introduction to what follows in the paragraph. It gives you the central idea, so, when you are looking for something, by reading the first sentence you can often tell if you are looking in the right place or not.
Well, as helpful as topic sentences are in reading, they are also extremely important in writing. In the band descriptors, under Coherence and Cohesion, Band 7 tells us that each paragraph must have a central topic. Assuming you have done all your brainstorming & organizing before writing, a topic sentence is a wonderful frame for making sure your paragraph is staying on track while basically providing an introduction to that paragraph for your reader. If you're not sure what a topic sentence should look like, take a look at various reading passages for IELTS. You'll see that the first sentence of each paragraph outlines for the reader what the paragraph discusses. Read some good sample essays. The topic sentence allows you to understand what you are going to read before you read it. Plus, it shows that the paragraph has a purpose, a "central idea," which you are introducing in the first sentence of the paragraph.