It gives a Linux experience within the security sandbox from Android, which is why there are limitations when using it on regular devices, via PlayStore.
Also it requires working around the NDK limitations on available POSIX APIs.
Linux is not userspace. Linux is a kernel, and android is a parallel fork of the kernel. So android is a version of linux but is not GNU/Linux.
Android/Linux, if you will. We dont have to be shy this.
GNU/Linux, the Linux desktop, has zero relevance on Android.
No one, not a single person, buying Android tablets and phones at places like MediaMarkt, cares about GNU/Linux Desktop.
They care about what apps via PlayStore, Samsung, Huawei and Xiomi stores, they can use, just like all normies.