Physics is worth knowing because it is beautiful. It is the hidden secret of the scientist. We may claim to be researching some topic or other because it is "useful to society" or it will revolutionize some technology but, more often than not, we are simply fascinated by some small detail about how the world works and we can't stop thinking about it until we understand it better. We are constantly astounded by the way a few basic principles work together to explain so many different things, and sit in wonder and awe at the beauty of the world. Like an artist, I want to share this beauty with others. I want them to know what it is to see through my eyes.
Unlike crackpots, who continue to make grandiose claims of finding the "theory of everything" and solving the question of the universe, the overwhelming majority of physicists really do not have such pretentious goal. We tend to want to solve a particular problem, or work in a particular area, because these are the ones that peaked our curiosity, and we want to find the answer to their questions. We just want to add to the huge body of knowledge, even if it is just a small portion of it. Of course, some of us are more aware of the application of what we do, and that certainly can be a motivating factor in addition to the curiosity angle.