As I expect you'll also have heard, the book is "a treatise on the relationship between us and the news media" and "a manifesto on the role of the press in American history". But I think both of those descriptions sell the reason for buying the book short.
The way I figure it, the problem a lot of people have is that they don't really understand media. Any of it. Maybe a vague notion about commercial interests or liberal bias or what-have-you, but little beyond that. I think people, on the whole, don't have any real media literacy. They don't see reporters as storytellers; they don't know how to judge/interpret what they're being told; they don't even understand the language well enough to discern why certain words were chosen for a report.
To some degree, I get it. Reading Marshall McCluhan is a tough slog. Trying to take hilariously obsolete opinions of new-fangled things like "radio" or "television" from their original time periods and relate them to contemporary concerns doesn't generally follow a straight path. The big picture is hard to look at, precisely because it's so big. Not to mention that a lot of people just don't even understand the basics of current technology. (Which is why phishing continues to work.)
But, at the same time... it's the 21st century, people! Regardless of what era you grew up in and how you'd like for the world to continue to operate as it did, that's not how it works now. A hundred years ago, "literacy" meant basic reading, writing and arithmetic. That's not enough any more. You used to go through life quite happily with a sixth grade education, but now it's difficult to just do that if you've got a college degree. "Literacy" has expanded considerably. Here's what Wikipedia has to say...
This idea [literacy in the 21st century] has forever changed the landscape of information access, and is integral in an understanding of Literacy as a practice, in the 21st Century. It is no longer sufficient to consider whether a student can 'read' (decoding text, really) and 'write' (encoding text), and it is necessary to consider more meaningful aspects of literacy in education and in society as a whole, if we are to complete the transition we are in, from a society in which communication was never possible on the level of 'many to many', to one in which it is.The Influencing Machine is essentially a primer on that notion of literacy today. It doesn't cover nearly everything that you need to become 21st century literate, nor does it strive to, but it does tell you what that literacy is and why it's important. That it happens to also be done so expertly makes it that much more critical that you read this book.