Is there any reason we use capital letters? I can’t think of any.
Clarity. It helps to mark logically significant places in a block of text - e.g. the beginning of a sentence - and to mark important words. As most of us read text by “skipping” our focus from point to point rather than by dragging it along in a steady line, this helps us to pick out some important reference points, and makes reading easier and quicker.
I believe they first came into play during the middle ages for the purposes UDS mentions. I don’t recall any lower-case Roman letters (letters from classical Roman times), but I believe they came in with Carolingian script and the flowering of the monastic movement.
Yes, capital letters came first, as a glance at any Roman monument will testify. Lower-case letters evolved later, as writing with pens on paper (or similar) became more common and there was a need for a script that could be written more quickly but still clearly.
…and inagine if we posted in all caps…