Loose sentence:
A sentence in which the main idea comes first, followed by dependent grammatical units such as phrases and clauses


Example:
"Halfway between West Egg and New York City sprawls a desolate plain, a gray valley where New York's ashes are dumped"  

-The Great Gatsby by F. Scott Fitzgerald

Explanation:

This is by far one of the most well known examples of a loose sentence. The purpose of this is to vary from the average sentence structure, adding interest to the passage. This adds a certain suspense to this long drawn out sentence. Also builds to importance of the setting and location of the novel.