When you are designing a project there are many common parts you may need a 3D model for. These could be things like screws, bearings, gears or many other commonly used parts. Designing these all yourself can wind up taking a fair bit of time and sometimes is not necessary. There are many websites out there like McMaster-Carr that supply a wide variety of parts and include 3D models of the item. These make it easy to either create the proper space in your 3D model for the part and then buy the actual part, or to use the 3D model as a file to print from.
You will often see a variety of file types such as a STEP file which is easily opened in 123D design. The file is often editable and can even serve as a framework for a slightly customized version. A great example of such a part would be this Nylon Sprocket. These parts catalogs with the 3D models allow you to conveniently browse available parts, while making it very easy to incorporate them into your designs.
Although there are sites like Thingiverse with many assorted 3D models, these parts catalogs will provide a more useful database of parts. Hopefully through the use of these online parts catalogs you will be able to save yourself some design time and finish your projects even faster.