Where are the options for this question? I'll just have to generalize from my own knowledge, I guess.
Sentence variation, meaning that the sentences are not always the same length as one another, brings great variety to paragraphs, essays, and stories.
The use of varying subjects keeps sentences flowing with more ease and they make them sound a lot nicer. Refrain from always starting with the same exact subject. If you read sentences like that nonstop, it sounds ugly, trust me.
These are the main points that I could come up with, and also the most important.
Hope it helps :)