It’s all about emphasis. The right sound standing out from the rest of the cohort. The right word stepping forward from the line of duty. The right phrase defocusing the rest of the paragraph. Emphasis is a leading principle in pretty much everything, and particularly in publishing. It wouldn’t be crucial in typography today, if it hadn’t become essential in the age of manuscripts.
As contrast and outline are essential for making out shapes and figures in the real world, so is emphasis for the purpose of identifying significant elements on a page.
Ancient and medieval scribes developed ways to emphasize text on a written page and to guide the reader’s eyes to important bits of information.
Boldface is a type of emphasis which came pretty late to the game. It wasn’t until the 19th century, the Industrial Revolution and the advent of advertising that words started to be written in bold. Although the use of bold fonts is recent, the principle is quite ancient. In the age of scribes, there were many ways to highlight words – either by writing them in a different colour, usually in red (we speak of rubrics from the Latin ruber meaning red), by writing them in a different style (larger or uppercase), or simply by crossing them through with a red dash (the ancestor of the highlighter).
Words stand out on their own or they are helped by emphasis devices. The fortune of picking up and taking away key words, phrases and ideas from a text is in fact being able to see. And how will it be seen if it hasn’t been emphasized? The regular and the bold need each other.