It has taken me some time, but I have realized now what most physicists do at some point in their life: you can not possibly become a functioning physicist without having a strong hold on mathematics. Yes, experimental and theoretical alike! One might think that understanding the laws of physics must serve one well, but while that is the necessary condition it surely isn't a sufficient one (Pun #1). While understanding physics well itself requires a lot of mathematics, researching in physics is a whole another ball game. I say this because when you are involved in researching you attempt to expand the boundaries of our knowledge, and that, at least for a theorist, requires extreme mathematical rigor. Why? Because mathematics is the only tool known to mankind that one can use to justify one's hypothesis. You make assumptions based on the facts you already know (or believe) are true and based on those assumptions you derive the mathematical form of your idea. And it doesn't end there, your theory isn't proved until it can be proven experimentally, ask Peter Higgs! I had started this post to express how important mathematics is for a physicist, and have drifted to telling you about the plight of a theorist. What a subtle distraction indeed.
While physics answers the 'why' behind a physical phenomenon, mathematics answers the 'why' behind the formulation involved in answering the former 'why'. For example, while you might know about how quantum mechanics involves Hilbert spaces and Dirac notation and is able to precisely explain the quantum world, functional analysis tells you why use Hilbert spaces, among all others, in the first place. Mathematics is like a tool box a physicist must have, and knowing more doesn't hurt because you never know what tool may be required to solve what problem. When I was in secondary school, I always felt that matrices were a lame topic. You arrange numbers in a definite form to ease calculations. Only in college did I understand the vastness of the field. Think of it, matrix algebra, linear algebra to be general, is one of the most important topics for a physicist as all the basic concepts and subjects one learns involves these bad boys.
And complex numbers, don't get me started on complex numbers. I can say this without even an iota of doubt that they are the most beautiful part of mathematics I have yet encountered (Pun #2). If you think about it, it is a very simple idea: what if we took the square root of -1? And there you have it, i, a letter that changed how people looked at the world. Moving from imaginary to some real stuff, no matter how much we get accused of being day dreamers, physicists work towards explaining things that are real. While we might get rigorous with complex numbers, at the end of the day, real numbers are what we return to. This itself makes it very important for a physicist to understand real numbers, and real analysis sure helps a lot in it.
I know what you are thinking, three paragraphs in and I haven't mentioned calculus! Well, do I need to? Calculus to me feels like a mathematicians' version of a theory of everything. Before you say it, I know calculus can not be used everywhere, but I say this because of the immense applicability of the subject. Well, I am sure Leibniz and Newton will be proud.
Sometimes people claim that physics is just applied mathematics. While this may sound tempting to mathematicians, I will have to side with the physicists here. I know that a mathematician can very well derive the same laws that a physicist did and in probably a more elegant manner, but a mathematician might not be able to appreciate the meaning behind that law like a physicist can. The difference in what they appreciate is what differentiates between the two most amazing people in the word: the mathematician and the physicist.