Designer here: there's a trade-off between visual harmony (all icons look the same) and ease of differentiation.
A standardized container adds regularity to irregular shapes.
Recently, Apple has been heavily opting for visual harmony, so their icons look consistent when seen as a set. Google too. It's an industry trend that is fairly annoying.
Non-designer here: The bounding container being consistent signalizes "this is an App," which is helpful in the broader context of an operating system. For example, if I saw this on my file browser, I'd have to think if it's an App or a document: https://upload.wikimedia.org/wikipedia/commons/thumb/9/9e/Li...
That first level of signalization builds on top of familiarity with iOS. The squircle signifying app shows up a lot, even in marketing materials for iPhones and iPads.
Once you're past that first level, you can use the shape inside the container. The Phone and Messages icons are just green squircles, right? Yet they're very distinctive, because the interior shape (phone handset, bubble) is what registers. https://t3.ftcdn.net/jpg/17/71/51/32/240_F_1771513287_ATNuUv...
Mac OS and iOS thumbnails documents/files for icons on the file browser and search, so shapes are irregular. E.g. Landscape thumb for a .pptx, square album art for an .mp3, portrait for a .pdf, arbitrary shape for a .xlsx. 3rd party apps can participate in that through Quicklook plugins.
It's a great feature because I can scan several files named export (n).xlsx on my downloads folder stack, and know which one I want from the thumbnail alone. OS feature improvements change the design context.
But is it simply trading actual concrete functionality and usability in exchange for the concept of "superficially looks nicer to certain people in a marketing image" ?
>A standardized container adds regularity to irregular shapes.
Does putting differently shaped icons in a standardized container make them harder to distinguish? When I look at an object its boundaries register first. If all icons are enclosed in the same square container, then they all look like squares at first glance.
A standardized container adds regularity to irregular shapes.
Recently, Apple has been heavily opting for visual harmony, so their icons look consistent when seen as a set. Google too. It's an industry trend that is fairly annoying.
Similar "let's remove the differentiation" decision made for menu icons in macOS: https://tonsky.me/blog/tahoe-icons/