My system is completely from scratch: including hardware: KT7A-RAID board, AMD 1.13 GHz processor (now running comfortably at 1.35 GHz ;-), etc.
I built my machine as a learning experience some 2 years ago. Originally, I installed RH 7.0 - which was then used to build my first LFS: 3.3. Since then, I have rebuilt LFS a couple of times, to gain experience and also as LFS has evolved. For example, I just recently rebuilt my LFS system based on the "Pure LFS" hint (which is incorporated into the CVS tree of the LFS book, but not LFS 4.1). Needless to say, that old RH 7.0 is long gone.
The main motivation, for me, was to learn. But, I can really see the benefits of using LFS. Most (all?) major distributions are quite bloated, with things that most people don't need. So, I only have what I want. If I want something more, I compile it myself. Also, most distros have the packages compiled for i686, i.e. Intel. While AMD uses the Intel instruction set, there are differences in the chip architectures. I wanted to get my system optimized specifically for me. The only way to really do that is to (re)compile everything yourself.
Maybe I'll host my own website soon, if I can figure out how to configure my router... :-)