There are three aspects to the Steampunk idea which make it an interesting sub-genre of literature. Though it's become more famous for its obvious aesthetic appeal, exciting the desire to dress in Victorian finery as well as the desire to have or at least to approximate geekery and gadgetry at the same time, the speculative fiction is the more interesting.
THE STEAM PART
1. It is a vigorous demonstration of the most famous Maxim of Marshall McLuhan
McLuhan once said "The medium is the message", and most people have totally misunderstood what he meant by it. He did not use the terms media and message in their most understood sense, but instead used them to represent a technology or idea, and the social impact or advance in other fields it allowed for. To give you an example: the electric light made surgery much safer. Candlelight was insufficient for this purpose, relying on the sun to provide enough illumination is a crapshoot, and burning torches produce soot and heat, and the chance of catching volatile gases on fire. Therefore one of the messages of the medium of electric light is the capability to perform surgery in a safer and more sterile manner.
The Victorians were decidedly and profoundly affected by the technology of telegraphy, which brought the world closer together, made easy and fast communication across great distances possible, but was also a source of great consternation and societal concern. They would not have been surprised at the impact of the World Wide Web - they went through that revolution themselves. Young uneducated punks with an in demand skill getting overpaid and then getting wiped out in a crash of said industry... modern day HTML programmers in the dot com days? Yes, but also telegraph operators in the 1800s. Likewise, there were strongly worded employee/employer communications about the abuse of the telegraph system for personal use, and there were several well known cases of women running off with men they met and chatted with over the telegraph lines.
The genius of putting the notion of a telegraph internet in place in these books did two things: it made us look at a technology and its messages in a novel way, having become accustomed to them in daily life. In fact, having to transpose modern conveniences and ideas into archaic technologies makes us have to re-evaluate the messages of the technologies we have instinctively taken for granted.
Also, if you think about it, a technology implementation usually prevents a later technology and/or a better technology from implementing same. Had we been able to machine gears to the tolerances required for computing, would we now have silicon chips? Some have speculated that our present use of silicon chips has been a bad thing, and we really should have waited until we understood photonics.
2. It's a casual reminder of how ephemeral things are
In the Victorian era the currency of energy was coal, Britain with its naval power was the biggest empire in the world. London was the capital of the universe and the United States was an ascendant backwater. The fashion of the time was over-ornamented frippery, and social mores were ostensibly puritan and muscularly Christian. It's a nice refresher course on old geopolitics and fashion, with the added twist that we get to speculate as to their development.
Case in point - suppose that oil had never been used as a technology for cars, but instead we'd chosen ethanol as a fuel. It would have meant that Saudi Arabia would never have developed any kind of financial base, and Middle Eastern geopolitics would be of no concern to us. But it also would have meant a serious damper on our 20th century mobility and growth, because one would have required, for example, the equivalent of four times the size of the United States to grow enough biomass to satisfy ethanol fuel demand.
Even just in looking at implementation, the "what if" in technology can always be fun. The Kawasaki Vulcan motorcycle was designed as a "what if", the designers having been told to build what they thought the Indian Motorcycle Company would have produced had it survived. This literature does the same sort of thing. What if the Victorian Age had had the ability to distribute processing power as well as data? Just as the printing press made the printing of the Marquis de Sade's work possible (because no monk would have ever transcribed such a work, which was the previous production model), it asks what would have happened to political influence, social mores and the like had the messages of the technologies of the age been married to the messages of the technologies of our own.
THE PUNK PART
3. It is still news to some people that the Victorians were sick, twisted sexual puppies
The historical perspective of the Victorian era is one of propriety and where Oscar Wilde was jailed for homosexuality and where table legs were covered so as not to inflame the passion of the menfolk. It was also the age in which child prostitution flourished, there were tons of clandestine spanker's clubs and vice rings regularly brought in top members of the clergy. The horror writing of the age hinted at the darkness behind the veil, with Dracula being a metaphor for loss of virginity and sexually transmitted disease, and Dr. Jekyll and Mr. Hyde being the fear of having one's darker baser nature discovered. Doing the whole film noir trip of seedy underworld types and gritty street cops banging whores and doing drugs revisits and underlines the fact that there's really nothing new under the sun in those terms either. In fact, the Difference Engine contained some wonderful scenes that highlighted Victorian sexual and gangland slang.
Whereas a lot of folks have come away from Steampunk with a yen to make brass framed welding goggles and coal-fired zeppelins, modding their computers with typewriter keys and suchlike (and there's nothing more punk than that kind of can-do, DIY aesthetic), that would be overlooking the real value of speculative fiction. We need more genres like steampunk. Perhaps medievalpunk, edwardianpunk, atomic age punk, or what have you. And there's certainly more to be mined in the many years of Queen Victoria's reign than the events in the Difference Engine. Here's to hoping the fertile geniuses of Gibson and Sterling come up with more.