All of these items can easily be created with artwork found online or on a computer. The average person should be able to locate all of the images needed to create such artwork in a few minutes and simply use Clipart to include them in their document.
Vector art is very different from bitmap art which is a photo or an image that can't be resized. Vector by contrast is made up of lines and points, so the program you use to edit it can change its size without getting jagged edges or making the artwork fuzzy. This is especially important if you're creating flyers and posters and want different sizes.
Free clipart isn't just a business, though; free clipart is a gift to businesses, especially small businesses. Letterhead, checks, logos, signs, and websites all benefit from free clip art. Whereas extensive design work once had to go into even the most mediocre of media, free clipart has helped build up the reservoir of design opportunities, affording entrepreneurs yet another way to take hold of some stopgap measures. While you wait to grow into bigger advertising projects, clip art makes your fliers exactly what you need.