It dates to the medieval copyists---mostly monks---who developed new ways of writing that were faster. In some cases the letters connected (what we'd call curs

It dates to the medieval copyists---mostly monks---who developed new ways of writing that were faster. In some cases the letters connected (what we'd call cursive), sometimes just flowed (italic), sometimes they were just shorter and rounder (uncial/half-uncial). The dominant form was this new style. BUT, for the beginnings of chapters and/or pages, they would revert to the old Latin letters, often with a great deal of decoration. Thus the capitals remained in active use even as the running text evolved. The printing press narrowed the number of distinct styles somewhat, but even today if you look at handwriting across Europe, the capitals are pretty recognisable and would look familiar to an ancient Roman, but the rest of the text diverges a lot. ...which doesn't exactly answer your question of what it's "for", I guess. But to some extent, we have two cases because that's what history gave us, and now we make use of them as best we can.