Hi,
Thank you for interesting to use this plugin. This plugin need 2 different API Keys because it will be used in 2 different purpose. The Browser API is used when you setting the Admin Area to get your store location info. And the Server API is used to calculate the distance for customer address and your store location. As Google does not allow you to specify both an HTTP referrer restriction (for the admin map usage) and an IP address restriction (for the server-side distance matrix calculation) so need to split it in 2 different API keys. The version 1.4 and earlier is just using 1 API Key, which mean that the API Key cannot be restricted at any type. And many users complaining about security as the API Key cannot be restricted.
I hope this the answer of your question.
Sofyan