Once you have established your goal and identified your target audience, you should take the following steps to construct your presentation.
Imagine this scenario: You are driving, trying to get from point A to point B. At the beginning of your trip, you see a sign telling you that you're going in the right direction. But then you drive and you drive. The road winds. There are detours and forks in the road, but there are no more signs pointing you towards your destination. Are you going the right way? How did you get to this point anyway? What is happening? Where are the signs telling you that the road has changed and you should go this way instead of that way?
In speeches, transitions serve this purpose. Take the road example from above and apply it to your speech:
Members of your audience will do their best to follow you as you speak. However, your speech may take detours. It is bound to move from one subject to the other. Will your audience make the correct connections or get lost? There is also the chance that members of your audience will drift off and when they tune back in will wonder how you moved from talking about X talking about Y. They do not know how you got there.
Proper transitions will not only eliminate such questions, but will also hopefully eliminate the situation in which members of your audience drift off. Transitions enable the flow of a speech. A speech without transitions often seems choppy, and can even seem unorganized. Using them allows your audience to follow your presentation. The words you use can lead them along, signal that you are moving from one point to another, or signal that you are stressing a point.
Source: Boundless. "The Role of Transitions." Boundless Communications Boundless, 23 Feb. 2017. Retrieved 21 May. 2017 from https://www.boundless.com/communications/textbooks/boundless-communications-textbook/organizing-and-outlining-the-speech-10/transitions-55/the-role-of-transitions-217-1059/