I don't understand what is the reason of buildroot of a card such as beaglebone black where as it has a Linux system when we bought.What is the utility of the buildroot?
Build a completely custom, lightweight, and fast-booting embedded Linux system?
Buildroot is a set of Makefiles and patches that makes it easy to generate a complete embedded Linux system.
Buildroot can generate any or all of a cross-compilation toolchain, a root filesystem, a kernel image and a bootloader image.
It is useful mainly for people working with small or embedded systems, using various CPU architectures (x86, ARM, MIPS, PowerPC, etc.) : it automates the building process of your embedded system and eases the cross-compilation process.
The resulting root filesystem is mounted read-only, but other filesystems can be mounted read/write for persistence.
Although user accounts can be created, in practice almost everything is done as root. Buildroot uses no package manager.
Instead, package selection is managed through make menuconfig.
来源：https://stackoverflow.com/questions/31834365/what-is-the-utility-of-the-buildroot-on-beagleboneblack