Every operating system ever created tends to have its own boot loader. Installing a new operating system on a machine generally involves installing a whole new set of boot mechanisms, each with completely different install-time and boot-time user interfaces. Getting multiple operating systems to coexist reliably on one machine through typical chaining mechanisms can be a nightmare. There is little or no choice of boot loaders for a particular operating system — if the one that comes with the operating system doesn't do exactly what you want, or doesn't work on your machine, you're screwed.
While we may not be able to fix this problem in existing proprietary operating systems, it shouldn't be too difficult for a few people in the free operating system communities to put their heads together and solve this problem for the popular free operating systems. That's what this specification aims for. Basically, it specifies an interface between a boot loader and a operating system, such that any complying boot loader should be able to load any complying operating system. This specification does not specify how boot loaders should work — only how they must interface with the operating system being loaded.