Aside from XDP (Xara Designer Pro) that I used in the previous screen shot, PagePlus & Affinity Designer also slant the cursor according to the font's numerical setting. MS Word also slants the cursor. This screen shot is from AD:
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I would heartily recommend AD (Affinity designer) for font testing. It is relatively inexpensive and no matter how esoteric of a feature I write in a font, it handles them all properly. When it was in beta, there were issues that I worked with Serif to correct and they were within a beta release or two. PagePlus is also good for testing fonts (and, well, the other things it is good for as regards layout work) and is very inexpensive because it will receive no further updates. But AD is modern, current and its OT Feature support is better than PP.
But yes, that is all that setting does. As you are experiencing with your current software, it can mean nothing. Even if I set it at zero, applications that do use that setting (like I listed above) would work just fine. So whether you use it or not is up to you. I find it a pleasant touch when using an application that utilizes that setting.
As for various angles in a font, and if one desires the angle setting to be non-zero, I would place and rotate a guideline on representative glyphs and make a choice re the angle to use.
Mike