If all your sentences start in the same manner, or if they have an inclination to all be roughly the same length, it can create a rather plodding, even dull kind of effect. Readers notice tautology, so using a variety in sentence lengths aids in making the text interesting and avoids making it too monotonous. By varying the length of your sentences, you can show the reader that you are thinking carefully about what u are writing.
Using an assortment of sentence structures can make the text more exciting and engaging. You could use short sentences to make emphasis.