All the marks on a printed page—and a lot of the spaces—carry meaning. Even variations in the shape of some of the marks carry meaning. And the meaning of those marks and those variations has evolved over time, just like spoken language.
Just read this little history of italics. An excerpt:
Interestingly, italics weren’t originally used for emphasis. They were used separately to the roman until the mid-16th century (although the capitals were initially all roman). It’s unclear as to who exactly we have to thank for using italics for emphasis alongside roman type, but it’s usually attributed to either François Guyot or Robert Granjon in France. Since then, italics have become a staple part of type families. Over time they have gained a number of specific uses, and this is where thinking about how they might be used in the content we are designing for can help us choose a good typeface.