Apple’s iPhone 5 isn’t just a new gadget. It’s the lifeblood of the world’s biggest tech company, representing half of its sales and even more of its profits. And introducing the new iPhone — getting good buzz and getting people excited to buy — was a crucial part of the puzzle. Here’s why September 12, 2012 was one of the most important days in Apple’s year.

Since launching in 2007, the iPhone has grown rapidly. A business that barely existed five years ago is now easily Apple’s biggest, representing half or more of the company’s sales, and an estimated two-thirds of its profits.

However, as the iPhone 4S has matured, the iPhone business has rapidly decelerated. During the June quarter, iPhone sales increased just 22% year over year, compared to typical growth rates around 100% over the past two years. That’s why it’s time for something new and attractive.

The new iPhone isn’t just important to Apple’s financial performance. It could also help Apple continue to chip away at Google’s lead in the U.S. smartphone market. Recent comScore stats show Apple growing faster than Google over the past six months.

Here’s one more fun fact. Last quarter, Apple’s iPhone sales were almost as big as Microsoft’s entire business. Last March the iPhone, in fact, passed Microsoft.

Here’s how the iPhone business stacks up against some of tech’s biggest companies, either reflecting their June quarter sales or the most recent overlapping quarter.

To sum up: This announcement is huge.