Well, for starters, knowing and being able to speak Mandarin immediately shortens the distance between you and local people, possibly your business partners. Many tasks are a lot easier and much safer to be carried out by you other than your partner or staff, such as opening an account at a local bank or registering your business at the bureau.
In terms of information access, you don’t have to rely on others. And if your Mandarin has reached a certain level, you may even be able to write and sign a contract without the help of other people. If such legal papers were handled by people other than you, usually more risks are involved. Knowing the language would save you quite some time and reduce risks.