Even one makes a big difference
A comma is used to separate items and it adds a pause to the sentence. How much of a difference can one make?
A panda enters a cafe' . He orders a sandwich, eats it, and then pulls out a gun & starts shooting. When a waiter demands an explanation for the panda's unusual behavior (fortunately no one was killed), the panda hands him a poorly punctuated wildlife guide booklet. The waiter is asked to look up the description of a panda. It reads: "Panda. Large black-and-white bear-like mammal, native to China. Eats, shoots and leaves.
Let's eat, grandma.
Without the comma it would be "Let's eat Grandma." Use the comma for the Introduction part of the the sentence.