I agree that it doesn't need to exist, but as far as I can tell, almost no one depends on it directly. The only person using it is the author, who uses it in some other small libraries, which are then used in a larger, nontrivial library.
I just created a Next.js app, saw that `is-arrayish` was in my node_modules, and tried to figure out how it got there and why. Here's the chain of dependencies:
next > sharp > color > color-string > simple-swizzle > is-arrayish
`next` uses `sharp` for image optimization. Seems reasonable.
`sharp` uses `color` (https://www.npmjs.com/package/color) to convert and manipulate color strings. Again, that seems reasonable. This package is maintained by Qix-.
Everything else in the chain (color-string > simple-swizzle > is-arrayish) is also maintained by Qix-. It's obnoxious to me that he feels it is necessary to have 80 different packages, but it would also be a substantial amount of effort for the other parties to stop relying on Qix-'s stuff entirely.