A lot of people simply don't enjoy writing--and they do their best to take any shortcuts they can find to make the process shorter. One shortcut is avoiding outlines. Outlines can seem like just another cumbersome step. Why not just get the words on the paper and get the thing done?
But outlines serve at least two purposes: generating ideas and organizing the content. Many writers experience some form of writer's block. That empty page is intimidating, the clock is ticking, and the brain...freezes.
Outlining can help unblock things. It's easier to write down a few main ideas and some supporting facts than it is to come up with complete sentences and paragraphs, after all. Start with the introductory paragraph and write at least a fragment with the main idea. For beginning writers, it may help to highlight this to remember that the whole paper should support this focus.
Generate a few more ideas related to the topic. These might be steps in an argument, for example, if you're writing a persuasive paper. They could be examples. They could be several different aspects of your subject, each of which provides more information.
Flesh out the outline with even more detailed examples that will go into each paragraph. By the time you get done with your outline, you should have nearly all of the content for your paper. This is a good time to read through what you have and see whether it has a good, logical structure. Do the ideas all relate back to that main idea? Can you see how you'll get from each paragraph to the next (i.e., the transitions)?
Once you're happy with the structure, all that remains is to write the information into full sentences and paragraphs and to make sure the transitions are smooth.
Trying to skip the outline may seem like a time-saver, but ultimately it may make it more difficult to come up with the content; and worse, you may wind up with a paper that has no real focus as you careen from paragraph to paragraph, trying to fill the page.