A brief question, which most, would likely answer just as briefly. One could expect answers along the lines of: why not; I enjoy it; to learn about any number of things; because I have to. If we expand on the thought behind the question slightly, it becomes a more profound philosophical question. Once expanded, the question could include aspects such as the relevance, importance and consequences of reading.
The first and likely most common reason to read is purely for pleasure. Presumably most will derive some enjoyment from their reading, regardless of other potential reasons. In this respect, reading provides an enjoyable opportunity for mental exercise, likely more so than most other ways of passing idle time.
Arguably the most important reason to read is to improve one self. Although it is possible to learn without reading, it has many advantages over other mediums. Through reading we have access to more information than with any other form of media. Reading in comparison to learning from other forms of recorded thought, such as television or radio, is by far the longest lived alternative. The possible exception to this would be open discourse, though this is problematic as human beings have a nasty habit of dying. Although this can be dealt with through the passing of information by word of mouth, it is likely to digress into a form of the telephone game. Reading allows us to experience the views of countless people through out the ages. We can have the most complex thoughts explained by the masters, be told the most entertaining fairy-tales from modern to ancient times or study the art of the most gifted individuals (never mind the infinite quantity of crap that has been spewed out over the eons.) Through reading we are able to experience all these things without needing to be in the presence of the authors.
The ability to read and write is one of the most important developments in human evolution. It offers a timeless window through which we can examine our history. It is possible that human communication may be able to surpass its dependence on written language, but at this point in time, it is unforeseeably indispensible. Is it not human nature to advance and improve? What better way can we hope to accomplish this than by building upon the millennia of recorded knowledge information and creating an endless supply of inspiration for the advancement of future generations.