Many analysts and investors may argue that Apple's (NASDAQ:AAPL) iPhone business is the tech giant's most important catalyst for growth. But a case could also be made that Apple's share repurchase program is even more important -- at least when it comes to the growth potential for the stock.

Twenty-two billion dollars to go
Of Apple's $90 billion authorized share repurchase program, the company has spent $68 billion so far. The impact on Apple's share count has been impressive, with total share count down about 11% since Apple first announced a plan to return cash to shareholders through share repurchases and dividends in March 2012. 

AAPL Shares Outstanding Chart

AAPL Shares Outstanding data by YCharts

To put the impact of Apple's aggressive share repurchase program in perspective, consider that 6 percentage points of the 20% year-over-year gain in Apple's fourth-quarter earnings per share compared to the year-ago quarter can be attributed to Apple's share repurchase program. Apple's share repurchase program is also likely one of the key factors behind the greater optimism in the market for Apple stock. Shares are up 44% in the past year.

With $68 billion of Apple's $90 billion program exhausted, Apple has about $22 billion left to spend before the authorized program expires at the end of calendar 2015. What kind of a dent could $22 billion put in Apple's shares outstanding at the current stock price? It could reduce shares by about 3.4% -- a small but meaningful amount for a company trading at 17 times earnings.

But will Apple increase its authorized buyback again?
There is a strong case that can be made for Apple to announce yet another boost to its share repurchase program this year.

First, as fellow Fool Evan Niu recently noted, at Apple's average rate of repurchasing $11 billion per quarter, Apple's funds for repurchases are set to expire at the end of Apple's fiscal second quarter, or the very timeframe that Apple has updated its program every year. Apple initially announced the program to return to shareholders in March 2012. And it announced subsequent increases to the program in April of 2013 and 2014.

Second, Apple's share repurchase program hasn't been aggressive enough to put a dent in the company's cash reserves. To illustrate, Apple's free cash flow, or operating cash flow minus capital expenditures, was about $94.5 billion total between Apple's fiscal 2013 and 2014. During this same timeframe, Apple returned about $94.4 billion to shareholders through repurchases and dividends. This means that Apple hasn't even had to tap into its massive hoard of cash and marketable securities in the past two years, which now stands at a whopping $158.8 billion. Its current level of share repurchases and dividends can be entirely supported by Apple's current levels of free cash flow. Apple isn't just a cash cow; it's a cash elephant.

Image source: Apple.

Other than the fact that Apple won't be getting as much value per share purchased with any buybacks executed today since shares are trading significantly higher than they were during the past two years, Apple shareholders have every reason to believe a significant boost to the company's share repurchase program will be announced this year. And given that Apple shares are far from looking overvalued, the company's higher stock price probably won't stop the company from authorizing yet another increase to the repurchase program.

If Apple continues to repurchase shares at $11 billion per quarter through 2015, Apple could reduce shares by about 7% during the calendar year -- not too shabby at all.

Apple's cash matters. Don't ignore it.