Price is negotiated upfront. Easier to get grants for specific projects than for more staff and equipment. Special formats handled. Can be confusion over copyright. They are not accustomed to library requirements.
Staff is trained in new skills.
Projects build and pave the way for additional digitization. Can start small and ramp up to more difficult projects. Make a title page, then publicize it. Have it auto start to the HTML title page. Many public libraries that might want to digitize documents has little or no budget for the process.