Proportional fonts use just enough space for each character, making them easier to read and avoiding the awkwardness that some characters in monospaced fonts have (look at the poor little lowercase m in the sample above—it appears smushed to fit into one character space
You might think, "Okay, but now characters won't line up neatly." That's true, but the upside of this sizing means you still retain the typewriter aesthetic with a bit more legibility. Notice the difference
You get the readability of a proportional font with the rough draft feel of a monospaced one. When coding, you get easily distinguishable characters and just the right amount of space for characters that need more or less
