There is a very, very large number of Linux distributions. Each distribution is built using the same basic building blocks but the end results are always different. The choices made by the distribution developers turn the building blocks into finished structures designed to meet a variety of needs—desktop, server, or some other specialized usage.
Learning how the various building blocks work together to create a cohesive Linux distribution is an excellent way to expand your Linux skills. And Linux from Scratch provides a challenging way for skilled Linux users to do that.