Tracking stocks appear when a large company spins off one of its divisions into a separate entity. The rationale behind the creation of tracking stocks is that individual divisions of a company will be worth more separately than as part of the company as a whole.
From the company's perspective, there are many advantages to issuing a tracking stock. The company gets to retain control over the subsidiary but all revenues and expenses of the division are separated from the parent company's financial statements and attributed to the tracking stock. This is often done to separate a high growth division with large losses from the financial statements of the parent company. Most importantly, if the tracking stock rockets up, the parent company can make acquisitions with stock of the subsidiary instead of cash.
While a tracking stock may be spun off in an IPO, it's not the same as the IPO of a private company going public. This is because tracking stock usually has no voting rights, and often there is no separate board of directors looking after the rights of the tracking stock. This doesn't mean that a tracking stock can't be a good investment. Just keep in mind that a tracking stock isn't a normal IPO.