If there's a semibold weight, why isn't there a semiitalic slope?
For example, if a font has a regular slant of 0 degrees and an italic slant of about 18 degrees, the semiitalic slant would be about 9 degrees:
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First of all, italic doesn’t exactly mean oblique (a.k.a. slanted). Italics originate from a specific form of cursive handwriting, which usually implies obliqueness, but most importantly implies distinct letter forms (see, e.g., this answer). You cannot really have a semi-italic in that sense, since, e.g., you cannot have a one-and-a-half-storey a.
But that doesn’t keep us from thinking about semi-obliques, which are technically possible as you easily demonstrated. The main point of italics/oblique as well as bold typefaces is to provide typographical emphasis, i.e., the reader should be immediately able to distinguish these from regular type and each other. Now, your regular reader will be able to make the following distinctions:
However, for isolated words or passages embedded in regular text, it is quite difficult to distinguish:
You thus cannot use both, oblique and semi-oblique, for different kinds of emphasis within the same text. As a consequence, you can always just use the oblique and there is no need to have a semi-oblique variant of a typeface.
But why do semi-bold variants exists then?
Bold (and light) words stand out from regular text as they change the type colour. Even a short glance at a paragraph will tell you whether it contains a bold word, while you cannot spot an oblique word that easily. This effect can sometimes be exactly what you want, but sometimes you may want to attenuate it by using a semi-bold variant.
You may not only want to change the weight of your emphasis, but also of your main text. And here, you may need a finer grading of weights, to get just the right level of intensity. For example you could use (for regular text and emphasis):
Unless your font family comes with optical sizes, you will likely scale it for headers (or footnotes). This inevitably comes with a change of stroke width that calls for different levels of font weight than emphasis within regular text. For example, a bold header is often too prominent, while a semi-bold one might just be right. In another example, footnotes with a scaled-down regular font may be to faint, while semi-bold is just right.
None of these applications of semi-bolds translates to semi-obliques.