Before the mid-1860s, Italy was a confederation of a number of independent states. These states were independent countries, loosely confederated for defense purpose (when they weren’t fighting with each other), each with ruling families jealously guarding their independence (which usually meant sources of income). It is one of the major historical debates of the modern period just what caused both Italy and Germany to unite a score or more political entities into two large countries. It is not in the nature of rulers to voluntarily give up sovereignty, and both Germany and Italy united within ten years of each other. My own sense is that world events were proving the traditional European model of tiny independent nation states (most were fragments of the Holy Roman Empire) was uncompetitive on the world stage. Britain was thriving because of huge trading markets. 
And the American model, where independent states united under a strong federal government (this fostered tremendous economic growth) was given much attention, especially considering the lengths that our country was going to stay united through our bloody Civil War. So the stamps of the Italian States were issued at the very end of the fragmented period of European statehood and so are not only of philatelic interest but are historical documents as well.
There are seven Italian States that issued stamps, with several hundred different issues in all. Here is the secret to collecting Italian States stamps: Yesterday we discussed the problems for American collectors of Australian States stamps (because the Scott catalog more or less regurgitates the Stanley Gibbons catalog listings, and these listings are in a format that is inconsistent with the rest of Scott). The same problem exists with Italian States except that the culprit catalog this time is the Italian specialized catalog Sassone. Sassone is an excellent catalog, with clear and concise listings, especially with minor varieties not being confused with major stamp issues (in case you haven’t noticed, catalogs listing minor shades or paper or perf varieties of common stamps as major varieties is a fierce objection of mine). But the problem with Sassone is pricing, and it affects not only Italian States but all of Italy proper. Though Scott makes pretence that they do their own market research for pricing in the Scott catalog, they never really have done so outside the US area of philately. In the case of Italian States stamps, Sassone prices are ridiculously high (in Italy, most stamps sell for under 20% of Sassone), and these prices are parroted in Scott making many of the Italian States stamps cataloging in the hundreds or thousands of dollars when they sell at a fraction of the catalog price. Scott (and Sassone) justifies this by saying the prices are for perfect stamps. But not one stamp in a thousand meet this criteria.
Overly high catalog prices are off-putting to philatelists. A collector looks at the catalog listing of Roman States, for instance, and sees numerous thousand dollar stamps. And then he turns to another country because he thinks that he can’t afford to collect Roman States. If he realized that nice looking Italian States stamps sell for 10%-15% of catalog, perhaps there would be more collectors of these issues. Forgeries tend to be a problem with Italian States. The reason is simple: after Italy united, Italian States stamps had no postal validity, and so no government agency was policing counterfeiting with the same intensity that they do when stamps represent a sort of government currency. But most forgeries of nineteenth century stamps are crude—having been made more as spacefiller replicas for collectors (most have been removed from the market in the last fifty years), and buying from reputable sources make for little risk when collecting these stamps. There are few areas of philately that give a collector as much a sense of classic philately as the stamps of the Italian States.