Use
The word is primarily used to describe people or intangibles that have no negative impact on their surroundings, such as "a benign wind" or "the use of explosives to create filler for the dam was benign."
Medical
The term is used frequently to describe the effects of cancers, when the said cancer is found to not be harmful to the person involved.
Colloquial Use
As a word, benign is not really one that's used in too many everyday circumstances; as such, it doesn't have very many common day usages.
Prose Writing
There are many instances where benign can be used in this type of writing as a descriptive term for what we would commonly call people or places that have an almost healing quality to them, which is where the word is generally used.
Warnings
To use the word properly, remember: It's not just for specifics but can be used to express yourself creatively and uniquely, like any other word. Choosing carefully where to use the word "benign" is the hard part.