Writers use short sentences to make text easier to read or to provide extra emphasis. Long sentences, which often contain multiple thoughts or ideas, increase the chance of a reader getting distracted.
The length of sentence used by a writer depends on the target audience, subject and purpose. It is rare for a text to be written entirely in either short or long sentences, but the appropriate ratio of the two types varies greatly. A book written for young adults, for example, may use shorter sentences than a complex scientific paper. Blog posts and speech handouts are also usually written with short sentences.
Short sentences are particularly common in advertising text. Copywriters know that the success of an advertising campaign relies on the target audience reading and remembering as much of the text as possible. By using short sentences, the writer reduces the chances of the reader getting distracted.
Fictional writers use a variety of sentence lengths. This prevents the text from being boring to read. Short sentences, which may only contain a few words, are used to break the flow of the text and provide extra emphasis. In some cases, the writer may place a short sentence in a new paragraph for even greater significance.