italics started because in 1500ish Francesco Griffo wanted to fit more words on smaller pages to make pocket editions of books, and slightly slanted letters let you fit more letters per line without making them smaller.
it looks different than normal text, and anything unusual in a block of text draws the eye. things that draw the eye are emphasized. and since italics became a standard but different font, it got used for emphasis
As the other commenter said, it was originally used to fit more words on pages. For emphasis, at first, italics were used to highlight foreign words or book titles. But over time, people realized italics could also be used to emphasize or stress certain words and phrases.
As printing methods improved, using italics for emphasis became a standard practice. Style guides started including rules on using italics to draw attention to important text.
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