Type Talk – Video History of Type
Typefaces were originally designed for easy readability, since the first typeface, Blackletter, was quite difficult to read on paper. Therefore, new typefaces such as Roman, Calson, and Baskerville were created. Compared to Blackletter, Roman uses straight lines and curves, which makes reading much easier due to its generally wide and profound appearance. Although I find serifs to be an interesting trait in the beginning phases of type, I initially thought it was just a design choice to make it look elegant and pretty. However, I soon realized that the purpose of the serif is to help distinguish each letter which again, pardon my repetition, achieves that goal of readability.
Great that you observed that serif typefaces were developed and designed to be functional.
The typefaces that were created (over time) started with humanist serifs like Jenson (and Garamond). Jenson, Caslon, Baskerville, Bodoni, and the sans serifs all have roman fonts (and italic as well).