Prior to BitBake, no other build tool adequately met the needs of an aspiring embedded Linux distribution. All of the buildsystems used by traditional desktop Linux distributions lacked important functionality, and none of the ad-hoc buildroot systems, prevalent in the embedded space, were scalable or maintainable.
Some important goals for BitBake were:
Handle crosscompilation.
Handle interpackage dependencies (build time on target architecture, build time on native architecture, and runtime).
Support running any number of tasks within a given package, including, but not limited to, fetching upstream sources, unpacking them, patching them, configuring them, et cetera.
Must be linux distribution agnostic (both build and target).
Must be architecture agnostic
Must support multiple build and target operating systems (including cygwin, the BSDs, etc).
Must be able to be self contained, rather than tightly integrated into the build machine's root filesystem.
There must be a way to handle conditional metadata (on target architecture, operating system, distribution, machine).
It must be easy for the person using the tools to supply their own local metadata and packages to operate against.
Must make it easy to collaborate between multiple projects using BitBake for their builds.
Should provide an inheritance mechanism to share common metadata between many packages.
Et cetera...
BitBake satisfies all these and many more. Flexibility and power have always been the priorities. It is highly extensible, supporting embedded Python code and execution of any arbitrary tasks.