The origins of this idea go back to the mid-20th century and modernism as it was applied to design. It is probably not surprising that when people first begin designing type they also consider this approach. Indeed, when a web browser is asked for an italic in a CSS rule and there is no italic, it will simply slant the regular to create a synthetic or faux italic. The variable most commonly associated with italics is slant. In contrast, a less-different texture is often useful in situations where you are setting multiple lines, whole paragraphs, or even entire pages in an italic. This stronger effect is useful for highlighting single words or short passages of text. Greater intensity in this difference will mean that the italic is especially useful for creating a sense of contrast with the regular.
Instead, they are meant to offer a different texture than the regular. Italics are different from bolds in that they are not meant to appear to have a different weight than the regular. Italics are probably the most misunderstood style in type design, but they are also the style with the greatest potential excitement and fun due to the large number of variables for you, the designer, to play with.
