Why are some words not capitalized in titles?

I understand the how, but the why escapes me completely. After a decent amount of searching on the internet I can only find people talking about how to capitalize a title (or more importantly which words to not capitalize). Even the style guides have been mute about why they are asking us to use such a convoluted set of rules.

Is there some typographical or grammatical reason for some words to not have intial caps in a title (other than the arbitrary "because that is what is done")?

The idea behind many title capitalisation rules is to emphasise key words of the title which apparently improves the "impact" on the reader.

From Wikipedia's article on letter casing:

Among U.S. book publishers (but not newspaper publishers), it is a common typographic practice to capitalize "important" words in titles and headings. This is an old form of emphasis, similar to the more modern practice of using a larger or boldface font for titles.