The multipart masks is a good example, another is one or more layers in a group each with styles for adjusting hue/contrast/background-pattern/gradient/border/s

The multipart masks is a good example, another is one or more layers in a group each with styles for adjusting hue/contrast/background-pattern/gradient/border/shadow etc. in a non-destructive way, the introduction of layer styles in combination with good vector support (and smart objects) completely changed how people worked and literally defined how the web looked & functioned for a good few years afterwards (a lot of 'Web 2.0' is just interesting applications of layer styles in Illustrator and Photoshop) Being able to maintain several 'versions' of the same image just by showing/hiding layer groups while still being able to edit & adjust things fluidly is a crucial feature in Photoshop, with GIMP you end up making copies of copies of copies and if anything needs adjusting or re-composing in the source layers you'll have to go back and perform the same sequence of steps and discard most if not all of the layers you made. Yes, you have 'make selection from curve' and other tricks in GIMP to maintain your masks as vector data, but it's an awful lot of trouble and is a laborious N step manual process after every change. Layer styles were basically CSS, for complex raster data, and it works beautifully.