The English language is organized and requires practice to get it right. Different cultures create confusion when it comes to proper punctuation and grammar. The comma is used when writing your thoughts down for others to interpret correctly. Here is an example: "Brian, who is such a critic, is always getting himself into trouble." In this sentence there are pauses that have to be present, so we use the comma to show those pauses. The purpose is so the sentence is read like it is spoken. This helps the reader comprehend the dialogue between parties clearly.
For more information on comma use visit: http://www.grammarbook.com/punctuation/commas.asp