To change the oil in a car, you need to know where the oil pan is located.  To change the spark-plugs in a car, you must know where the spark-plugs plugs are located and what they look like as well.  You get the picture.  A mechanic must know the parts of the car before he or she can correctly service the vehicle or address problems that prevent the vehicle from functioning properly.

In writing, a writer must be able to identify the parts of a sentence in order to address problems that prevent the sentence from functioning properly.  Just as the mechanic would be foolish to try to work on a vehicle without knowing the parts and their locations, a writer would be just as foolish to try to revise sentences without knowing the parts and their locations.

Think of the various components of a vehicle as the parts of speech in writing.  All the parts of a vehicle work together to keep the vehicle functioning properly just as the parts of speech in writing work together to keep the writing functioning properly.  Certain components of the vehicle keep it moving along, stopping and starting in the right places, and pausing whenever necessary; punctuation does the same for writing.

To appraise the analogy, learning the parts of speech, the grammar rules, and punctuation rules are vitally important to becoming good writers.  The entity is known as the mechanics of writing.