This is great until one of your rivals steals a trade post from you. The more trade posts you have the more money comes in. You're bringing in a bunch of money, and spending it on building new trade posts to expand your trading empire. These are pretty resilient, weathering invasions and counter-invasions and making money the whole time. This is so much better than vanilla CK2, where there was never any money and you had to sit around forever before you'd saved up enough to build anything! As a bonus, rather than territories, your primary bases are trade posts.
You'll start off as the Doge of Venice and the money will just flow right in. It may seem like this would be a good position to be in. Most of all it meant that the republics had a bunch of money. Seaborne trade encourages different technologies: not just better boats, but navigation, governance, financial instruments, and naval warfare. This made for a lot of travel by boat for long distances. The basis of the merchant republic economies, on the other hand, was trade. This made for a static society where most warfare was based on attrition, whether in the field or at siege.
In particular, agricultural land, the people who worked it, and all the technologies of husbandry, warfare, and transportation that came along with it. The basis of most economies at the time (in particular, the economies simulated by CK2) was land. In particular, the republics provided two vital components every crusading army required: money and ships. These republics (especially Venice, Genoa, and Pisa) played a large role in the success and failure of the different Crusades. It is the aim of this expansion “Crusader Kings 2: The Republic” (CK2:R) to add these vital players to the game. Given this focus it was odd that the original game did not allow one to play as any of the merchant republics. One could always play as Ireland or another minor power in hopes of avoiding getting drawn into the titular conflict, but any nation of any size is going to hear the call of “Deus Vult” (or “Jihad”, depending on which side you're on) The geographic scope is also limited to Europe and the Middle East, the major players in the Crusades. “Crusader Kings 2” (CK2) covers, not surprisingly, the time period of the Crusades.