Transitions are usually put in as an afterthought. But keeping transitions in mind can help you organize your presentation before you begin. Suppose you were putting together a presentation. One of your first tasks would be to make a list of the ideas you want to include. Take this list, for example:
The subject of this speech is how to make a fruit salad. If you take the first two items on the list, oranges and apples, it's easy to see how you could transition from one idea to the next. It would also be a logical step to the next idea, fruit salad. But what happens when you come to "haircut"? You'd have to travel some awfully winding paths to get to that topic from fruit salad. If you can't easily make a transition from one point to the next, it means there's something wrong with your organization—it might even mean the point doesn't belong in there at all.
If we take haircut off the list, we've got: oranges, apples, fruit salad, pears. Although you could do it, you might have a bit of trouble moving from the combined fruit salad back into the solitary pear. It would be easier to transition from apples to oranges to pears, and finally to fruit salad. So the most logical order of ideas for this presentation would end up being: apples, oranges, pears, fruit salad. Considering the relative ease or difficulty of transitions before you put your presentation together saves you a lot of time that would otherwise be spent in editing and rewriting.
This fruit salad list is a simple example, but one that shows how you can use the concept of transitions to help you construct a clear, logical, easy-to-follow presentation.