While no account information, authentication and other stuff can be obtained by the key alone, it's used to link API calls to a Google Project.
If the key is publicly available people could do requests with it and the Google API would link the requests to your Google Project. This could give malicious users the ability to spam requests with this key, probably making your account hit the daily request quota.
In the end it's up to you, but it's recommended not to make your API key public.