Capitals tell you what's important. Beginning of a sentence makes it easier to read. In poetry, beginning of a line helps anchor the form, and also distinguish between forms -- haiku tend not to use capitals at all, which contributes to the soft, natural feel of that form. Proper nouns distinguish individual persons, places, or things from the general surroundings. Capitals also cause the mind to pause a bit and consider what's important, whether it's a new sentence or important noun. So if the text has too many capitals, the reading bogs down a bit; and when every word is emphasized, effectively none of them are. That works in a one line-title or in a signboard, but otherwise not so much.