Before smart phones I often found it useful to add additional information after a phone number in a contacts database, such as an extension number (x123), person's name or initial (John or J), function (billing), etc. The template in Contacts does not have a field for Extension which would allow for this. In addition, with the iPhone, the number itself will not even assume the proper format (area code in brackets, 3 numbers, dash, 4 numbers) if there is any additional information after the number, and therefore you won't be able to dial the number.
Putting a comma after any phone number (read as a one-second pause, as with modems) allows for any such qualifying information to be added. For me this is much easier than creating hundreds of custom fields for phone numbers.

