With a huge amount of people using small screen devices to view the internet on a very regular basis, and that number simply growing year on year, designers have to have the knowledge, tools and resources to bring the people what they want and need.
Design agencies have to make sure their own portfolios are pretty much perfect. It is their point-of-sale, and prospective clients need to like what they see to make them take further steps towards using the agency.
In short, Git is a version control software. It means it helps manage changes to a project without overwrites and loss of work. Therefore, if something gets messed up, broken or overridden Git is going to help you get back the otherwise lost work.