Some English sentences are very basic:
Shakespeare was a writer.
Einstein said something.
The Inuit are a people.
You could write an entire essay using only simple sentences like these:
William Shakespeare was a writer. He wrote plays. It was the Elizabethan age. One play was Hamlet. It was a tragedy. Hamlet died. The court died too.
Stylistically, it would make for a rather boring read; although, you would receive 100% on grammar. This chapter helps you learn to recognise different types of sentences and to use them effectively in your own writing.