Capital letters give shape to lines of text on a page that make it easier for the eye to skim. Sure, they don't add any additional information that the period does not already have (and proper names are mostly vanity), but try reading page after page of all lower-case. It will take longer and comprehension will be lower. This is the same reason why some letters extend above and below the line - makes decoding words easier by giving them shape.

You can try it out yourself by saving this bookmarklet and using it to downcase a page! (Create new browser bookmark and paste "javascript:..." as the URL)

javascript:for (e of document.querySelectorAll('*')){for (c of e.childNodes){if (c.nodeType === Node.TEXT_NODE) c.textContent = c.textContent.toLowerCase()}}; void(0)

Come to think of it, the ease-of-read effect does look more pronounced on a full page of text rather than someplace where every post is a single sentence.