In short, the reason for the use of capitals in writing English, and other languages, is a tradition with roots on aesthetics, not on linguistics. As for the accuracy of what I said, it is a good idea, for those who may be interested, to study paleography, especially the medieval scripts. I hope I could have shed some light on the matter.

Porsche10x, scrictly speaking, a language does not have to do with its writing system. Languages like Turkish for instance changed their writing system from the Arabic-based Ottoman script to a modified version of the Latin alphabet just recently (1928). So it is correct to say that the modern convention for writing the English language employs in fact capital letters, but it does not mean that if you abolish them you will have a different language.

English does not have capital letters. The script it uses has. And the reason is tradition. The capital script was designed by sculptors in ancient roman times and acquired prestige since it was used in monuments. When written with a pen, it proved to be time-consuming and demanded a skillful scribe. Therefore, texts written entirely in capital letters were reserved for “de luxe” editions. The minuscule were developed to give comfort to the scribe and save on writing material (namely parchment), while maintaining legibility. So, they were reserved for “economy” editions. To add more dignity to the minuscule text, scribes began “decorate” it with the more prestigious capitals. So titles of books and chapters were written in capitals, as well as proper names. Also the first letter, and sometimes, the first word, or line of a chapter or paragraph. This practice receded with time to using capitals only for the first letter of words of a title, proper names or periods (from a full stop to another).