Distributed Version Control Systems (DVCSes) are becoming a bread-and-butter everyday tool for software developers. One issue with Unity projects can be the many binary files in each project. There are also many files in a local system's Unity project directory that are not needed for archiving/sharing, such as OS-specific thumbnail files and trash files. Finally, some Unity project folders themselves do not need to be archived, such as Temp and Library.
While Unity provides its own Unity Teams online collaboration, many small game developers chose not to pay for this extra feature. Also, Git and Mercurial (the most common DVCSes) are free and work with any set of documents that are to be maintained (programs in any programming language, text files, and so on). So, it makes sense to learn how to work with a third-party, industry-standard DVCS for Unity projects. In fact, the documents for this very book were all archived and version-controlled...