Simple, some medium to larger scale projects require a lot of materials and equipment, and when they do, we request a deposit to begin work. Your deposit allows us to secure these necessities for your project. Don't worry though, your deposit goes towards your final invoice so it's not an additional cost.
The amount of the deposit varies between jobs, but if one is required, you'll know, because it will be on your estimate.
If you do not pay the deposit, work cannot begin on your project.