Why would a company issue a stock split?
Sometimes, the stock of a company like Apple will climb so high that its price is hard to afford for many average investors. Rather than allowing high prices to discourage buyers, the company will issue a stock split.
When a company issues a stock split, they are literally splitting the shares of stock into pieces. For example, the recent Apple stock split was 7:1. If you owned 10 shares at $644 per share before the split on Friday, you owned 70 shares at $92 per share on Monday morning.
Since the pizza analogy has been beaten to death, let's use an apple pie: more slices does not mean more pie. The market cap of Apple did not change. Notice that there is no inherent value creation or destruction that results from the split itself: before the split you owned $6440 worth of stock, and after the split it is still $6400 worth of stock until trading starts on the day of the split.