Mechanical Drawings are a complex form of drawings which are done using tools like compasses, T squares, etc. Such tools allow us to get an accurate drawing which can be measured precisely. These could also be 3D drawings. The Mechanical Drawings that are made for your patent filing concentrate on every detail of the functioning of the invention.
They say a detailed drawing is worth more than a thousand words. Why Mechanical Drawings are imperative in a patent filing because even if you mistakenly leave out something from the written part, the illustration is sure to do justice to the invention.
Because a mechanical invention is quite complex with n number of parts and components, it is the mechanical drawing that conveys the proper information about the structure and function of the invention. Hence, you really need a professional illustrator to capture all the nuanced information of your invention in their illustration.