Cohesion allows your readers to read through your documents smoothly. It is important to have
cohesion because your list of ideas/sentences need to flow smoothly to explain and describe ideas properly. Therefore, good cohesion links your sentences and paragraphs together and so your ideas flow smoothly for quick and easy comprehension.
For example, if you are explaining and listing out your ideas, you want to show how your ideas are linked and related. Are you adding or contrasting ideas eg firstly, secondly, also, however, regarding, this, those and even though. You might also summarise and conclude your ideas in each paragraph. However, if there is no cohesion, ideas will seem muddled and disjointed.
Logic is the idea that sentences should follow a helpful order and be organised/arranged to make easier sense. (But logic also includes a degree of cohesion.) Logic helps readers understand by making sure there are no surprises or unexplained ideas/reasons. Ideas might also need to build on each other to explain or present information in a helpful sequence. Logic is helpful for persuading, negotiating or appealing to someone.