I'd bet that a reason the first line indent was invented was as a space-saving method of indicating new paragraphs. Do we skip a whole line and waste paper, or ... ? "I know! We can put a bit of extra space before the first word of the next paragraph!"
Such things are not required in digital media where each reader might have completely different spaces for text to render within. And, ideally, the content and its formatting are structured in such a way as to allow the user to apply their own formatting if desired.
warning & disclaimer: post may contain factually incorrect guesses and suppositions of the author; no information herein should be considered accurate or factual; use at your own risk
this is not strictly true, but it is a postwar convention that was popularized by Penguin books [1] and its 'composition rules' which were canonized by jan tschichold. The more common 'tradition', which Typeplate ignores, is that you indent only if you do not insert vertical space between lines. The idea being that you only need to do one visual 'thing' to make a block of text distinct from another, why do two?
the earlier tradition was that you'd add a pilcrow or some other distinctive mark at the start of a logical paragraph mid-line so that as little paper (which was either baby cow parchment or made of precious linen) was wasted on whitespace. For an example, look at a 42 line bible [2] or eric gill's book on typography [3].
