Why would a writer choose to ise a variety of sentences

To make the text less monotonous.

Explanation:

It's important for a writer to use different types of sentences, because sentences with the same structure tend to make the text monotonous and less interesting to read. For example, instead of using only simple sentences, writers should use complex and compound sentences as well, or instead of always starting the sentence with the subject, they could change the word order to make the text easier to read and more engaging.