When you go to write a letter you'll usually start by writing the address and date, then who it's to and then you start on the main paragraphs. Already you've written text in several styles. The address and the date are pushed over to the right of the page, and you've got a blank line between them. You've also put each line of the address on its own line. Now once you've started on the main paragraphs of your letter, you find that you're either leaving a blank line or an indent to separate your paragraphs from each other.
These conventions for identifing and separating the different parts of a letter have become standard and are recognised by most people. Unfortunately computers are a different matter. When writing on paper you have lots of space to play with and you can use blanks and the position of things to give clues as to what it means to the reader. All computers have to work with is the sequence of characters that you type into them; they don't even have as strong a concept of lines as they just use a special marker in the stream of characters to delineate lines.