In some places, city blocks are always referred to in terms of hundreds. For example, in Chicago it is normal to hear somebody talk about driving from the twelve hundred block to the seventy three hundred block of some street. This means going from a block with addresses of 1200-1299 to a block with addresses 7300-7399. It only works in cities with a regular grid layout where each street covers a range of exactly 100 addresses, but in that case the "hundred" is the basic unit, rather than the individual addresses themselves. So, you can sort of think of twelve hundred being grammatically analogous to twelve kilograms or twelve apples.
In general, I never encounter it in formal written documents. The only time I see it in writing is when somebody wants to explicitly capture a speaking style, such as an exact transcript, or dialog for a play. However, when reading a document out loud which says "7300," it can sometimes be idiomatic to pronounce it as seventy three hundred instead of seven thousand three hundred. ("Our Chicago office is located on the seventy three hundred block of Wabash street.")