GTA Online today looks a whole lot different from how it was at launch. Over 30 DLCs have been released, some big, some small, some very big, changing not just the amount of items in the game, but also how progression works. The result is a steady stream of new content for die-hard fans to dig into every now and then. But.
The other result is an absolutely batshit massive game which, as a newcomer, might seem utterly daunting. Reaching the content of the newest DLCs would take a fresh, out-of-the-box character circa 50-75 hours of gameplay, if not more, considering the prices of things and how each update is built upon the previous one.
Grinding your way up in rank and collecting all the money to jump into new content is a massive undertaking.
Sure, you can grind endlessly and save up to beeline to whatever the newest content is (currently The Doomsday Heist) and jump over the content of previous updates, but you still need to lay some foundations. One of the big gates locking away gameplay content is the executive office, which is the backbone of basically everything post mid-2015. They start at GTA$ 1,000,000.
Then, you need some specific vehicles to stay competitive, and to keep freshening up your garage as money allows. By the time you'd be halfway to your goal, another DLC or two would have been released further adding to the huge cost you need to get there. Basically, GTA Online isn't all that accessible for new players right now.

The Criminal Enterprise Starter Pack is Rockstar's solution to this issue.
GTA Online has already carried microtransactions for years now, however that only allowed players to purchase varying sums of in-game currency. They still need to either do research or play and gain experience and understanding before they know what to spend that cash on to advance.
This Starter Pack is also something of a guide. For $44.99, you get a massive package of in-game items, vehicles and properties, allowing you to access a much wider array of GTA Online's features and content, instantly. It also comes at a discount compared to buying together all the Shark Cards needed to cover the costs of these same items. Additionally, you also get a boost of GTA$ 1,000,000.
So what items are in this package, and how useful are they, really?
First up, you get a bunch of properties, some of which are absolutely crucial for content unlocks:
Properties