Outlines are just a practical way of handling less common display cases.

Just to give a practical example. Imagine a Star Wars advert that has the Star Wars logo at the top, specified in outlines because that's what every vector logo uses. Below it the typical Star Wars intro text stretched into perspective, also using outlines, because that's the easiest (display engine doesn't need complicated transformation stack), efficient to render (you have to render the outlines anyway), and most robust way (looks the same everywhere), way of implementing transformations in text. You also don't have to supply the font file, which comes with licensing issues, etc. Also whenever compositing and transparency are involved, with color space conversion nonsense, it's more robust to "bake" the effect via constructive geometry operations, etc, to prevent display issues on other devices, which are surprisingly common.