There is a story about a young mathematics instructor who asked an older professor "What do you say when students ask about the practical applications of some mathematical topic?" the experienced professor said "I tell them!"
About a few weeks ago, my graduate students and I were just chatting away while we were waiting for the start of our meeting, and the topic of our conversation meandered into dealing with the "applications" of physics, and the usefulness of various topics in physics. I indicated that, as someone who have been involved in hosting open houses, tours, Q&A, etc. with the public, I've had to deal with trying not only to explain certain aspect of what I did, but also why what I did was important and deserved to be supported. That was when one of the students asked me what I tell a general public if I was asked what are the useful applications of certain topics in physics. I found myself replying "I tell them!" Thus, the connection with what I quoted out of Mary Boas's text.
In my years of being a physicist, I have tried to impress upon people that physicists don't just do esoteric, theoretical, no-applications-in-sight-type of work. In fact, as someone who was trained as a condensed matter physicist, I tried to impress upon the public that many of the things they enjoy today, especially their modern electronics, are all part of the areas that physicists work in. I've pointed out earlier that physics is not just the LHC, but it is also the iPod and the iPhone.
Unfortunately, being able to "tell them" the applications of various aspects of physics requires someone who is not only knowledgeable, but also someone who has a wider view of physics, and the ability to make connections between many of the common conveniences, devices, procedures, etc. that we enjoy that came into being due to physics. Many students aren't that well-equipped yet on being able to handle such breadth of knowledge that tends to come about from experience. I have done this, and have faced this type of question countless times that I can pull things out of my back pocket in an instant. However, what if you are just starting out, or what if you yourself want to know how to deal with the question of the usefulness of physics?
There are 3 different points that you can make:
1. Many areas of physics have a DIRECT and OBVIOUS applications. Condensed matter physics, atomic/molecular physics, medical physics, etc. can all boast a very obvious connection to modern electronics, medical advances, understanding of materials, etc. So point these out first, and in fact, point out that people in these fields are the MAJORITY of practicing physicists. You'd be surprised how many in the public area actually shocked by that revelation.
2. The utility of knowledge. Even in areas in which it is difficult to argue for the application and usefulness to the public, we can bring out the argument that it is difficult know the future applications of such knowledge. The history of physics is littered with many such examples, including quantum mechanics. The early development of quantum mechanics had almost no emphasis on the usefulness and application. If we only want to fund work that had such clear utility, then we would have missed out on the development of quantum mechanics. And yet, as we now know, our entire modern conveniences are based on this amazing theory. So while there is certainly value in the knowledge itself, demanding that it must have an immediate and obvious application is extremely shortsighted and ignorant.
3. The "side effect" of esoteric knowledge. One may doubt the usefulness of high energy physics, or astrophysics. However, there is no argument on the fact that the pursue to verify various aspects of theories from these fields via the advancement in particle accelerators, detectors, and other devices have had significant impact on our lives. Modern facilities and devices, ranging from synchrotron radiation centers, medical accelerators, light detectors, high-speed electronics, advance data/computations, etc. were all developed or came into being due to the advances that had to be made for various high energy physics and astrophysics experiments. These found applications, what I called the side effects, in other areas, and into our everyday lives. So the experiments to study these "esoteric" topics actually drive the innovation in many different areas that eventually trickled down to us the consumers!
So if you are starting out in physics, or already knee deep into it, do not run way from the question of the usefulness and applications of physics. You stand your ground and you tell them!