When you are writing some text on the iPhone (any application) and you need to add a punctuation char (comma, semi-colon, full stop etc.) you generall tap the "123" button, tap the punctuation char, and then tap the "ABC" button to switch back to the normal keyboard mode.

There's a quicker way to do it though: press the "123" button and -- while you hold your finger down -- drag your finger on the punctuation char. When you release the finger the punctuation char will be added and the keyboard will still be in normal mode.