I once punned in a headline “No comma sense”.

This referred to a number quoted by a government department which did not know where to put the comma between noughts so it had created an astronomical number, millions more than what was accurate.

With numbers commas and their placing are critical. With words, commas are the most helpful grammar tool, to help make sense of a sentence.

The functional comma has no limits to its usage, but its use should be limited, that is, only used when necessary.

Essentially commas separate phrases to allow pause and to manage the sentence in bite-sized chunks.

Three main uses:

Lists: Use a comma to separate items in a list, but note: ‘Red, yellow, blue and green balloons decorated the welcome area’, needs commas. But ‘Big blue clown balloons made the children giggle,’ does not. If words are of a different class in a list, no comma is needed.

Introductory phrases:

Use commas when you introduce your sentence especially to denote time or space ideas.

• Yesterday, the rain poured down in buckets.
• In the summer, the children would swim every weekend.
• While running, John also talks on his mobile phone.
• Travelling towards the mall, the shopper checked to see how much money was in her purse.

Breaking up phrases or saying more about the subject:

To check yourself, take the words between the commas out of the sentence. You sentence should still work. If not, you need to move the comma.

• John, who graduated yesterday, is planning a night on the town.
• Universities, which will close for the summer break, will have a challenge determining fee increases.

A helpful hint is that these commas always occur in pairs.

• I knew Tony would win the race because her brother, the only other contender, told me she was super fit

This panda joke is a lovely demonstration of what the little comma can do.

A panda walks into a cafe. He orders a sandwich, eats it, then draws a gun and fires two shots in the air.

“Why?” asks the confused waiter, as the panda makes towards the exit. The panda produces a badly punctuated wildlife annual and tosses it over his shoulder.

“I’m a panda,” he says, at the door. “Look it up.”

The waiter turns to the relevant entry and, sure enough, finds an explanation.
Panda. Large black-and-white bear-like mammal, native to China. Eats, shoots and leaves.”

― Lynne Truss, Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation