A comma is said to be a pause in speaking. That is generally true, but one should look at the reason for the pause to understand when to take it. Only a pause that is needed to clarify meaning is justified.
Ex.: I need to buy socks, bananas, a toothbrush, and a hat.
Ex. Peter went home, and I went to sleep.
Ex. I left, and then all the trouble really started that night.
Ex. (WRONG) I hit the ball, and ran home. (Why? There’s no subject in the second clause)
Ex. Since the age of nine, Sarah has been a star soccer player.
Ex. Myron lives in San Francisco, which is my home.
Ex. Although I am straight, I have many gay friends.
Ex. I now own a Honda, my first one ever.
Ex. In Nebraska, people often watch TV.
Ex. Tom, my oldest friend, lives in Virginia.
Ex. The third President, Thomas Jefferson, lived in Virginia.
Notice the difference between these two sentences:
There is s subtle difference between the two; the first implies that there is only one life raft. If the boat is sinking, there’s one place to find the raft! In the second example, there seems to be more than one life raft. We’re only told about the one at the back of the boat, though. –In a fire at sea, the commas could save or cost you your life by helping you understand the details. If the second one also had this clause following it, the differences might be more clear:
...", but the life raft found at the front of the boat inflates by pumping it's orange pump."
(It might seem that all the information in each of those two examples is “essential.” But in the first example the part “found at the back of the boat” could be dropped out. Why, because this sentence is about how to inflate it, not where to find it. The added phrase gives you more information, but it’s not essential to this sentence. The first two examples have extra information that isn’t essential to their meanings but is helpful to understanding the situation. The same is true for the raft examples. – If your head is spinning at this point, forget the life raft examples and stick to the Virginia examples!)