Orwell in this essay explores the reasons that writers write, citing his own example. He looks back to his earliest writings, recalling how, as a child, he would write just for the sake of making up stories, and describing scenes, as children do. He examines how his writing evolved as he grew older, noting how, increasingly, his writing came to have a political dimension, especially after his unpleasant experiences of working as a colonial officer in the East.
In the second half of the essay, Orwell postulates four main reasons why people write.
Sheer egoism. Most writers, Orwell opines, are extremely vain and self-centred and so they want to write about themselves, to show what they can do, to make their views and opinions noted, and so on.
Aesthetic enthusiasm. People who write for this reason take pleasure in the beauty of the world and also of words, and they want to express this. They want to share experiences which they feel are valuable and to arrange words or a story in a way that is pleasing. This motive, according to Orwell, is 'very feeble' in most writers.
Historical impulse. These writers are interested in facts about the contemporary world, and want to record them for future generations.
Political considerations. Writers with this motive want to change society in some way, to alter people's perceptions, raise awareness and so on. Orwell evidently feels this is the most important purpose a writer can have, certainly in an age like his own, which was the era of momentous political revolutions in Europe, grim totalitarian regimes like Stalinism and Nazism, and world war. In such an age, writers really have a duty to engage with political concerns, Orwell opines. He feels that the political dimension has added real value to his own work, which otherwise lacked a proper purpose.
I would say there are two mains ideas in Orwell's "Why I Write." The first would be that one needs to know a person's background and development to understand why they write what they write. This is why Orwell spends the first portion of his text talking about his youth and some of his earliest works. He says, "I give all this background information because I do not think one can assess a writer's motives without knowing something of his early development. His subject matter will be determined by the age he lives in..." Basically, every story a writer tells is influenced by a story in the writer's life
The second main idea is that every writer is motivated to write for a reason. This, as Orwell states, is influenced by the world around them. According to him, the four reasons for writing are: sheer egoism ("desire to seem clever...be remembered..."), aesthetic enthusiasm ("Perception of beauty...in words..."), historical impulse ("desire to see things as they are..."), and political purpose ("Desire to push the world in a certain direction..."). He argues that each of these four motivations explain why someone writes, though they affect one in varying degrees.