Usually, this is the first region where the companies decide to market their products. The consumer needs and behaviors, various distribution channels available, government rules, laws and regulations in the home market are very clearly know to a company and hence deciding on the marketing mix is relatively easier. The competition in a home market is relatively less intense and the cost of shipping goods within the home market is minimum.
The main disadvantages for a company to focus only on its home market is that it has a limited market size (potential) and growth rate.
E.g. the home market for Apple is the United States of America since it is headquartered at Cupertino, California in the US.
Similarly the home market for Volkswagen is Germany.