I guess it depends where you want the knowledge. When LFS first got started (and a few years before), I found at that time as a home-user, I often had to build the kernel myself (maybe I was trying too hard to be cutting edge), know more about booting (LILO and GRUB), and on occasion I messed up bootable partitions. Recently, I was trying to build a custom version of GCC and dependencies and found LFS docs helpful.

From looking over the BLFS now, I haven't had a lot of use-cases for building more userland-ish things. I've tried to avoid compiling X or Display Managers and stick to package managers if I wanted to try a new Desktop Environment. I've found Arch docs (or man pages) really helpful in configuring these kinds of things when I do need it. I can see this being more useful if I wanted to be more of a sysadmin. I remember trying to customize PAM and setup Kerberos years ago and feeling like there wasn't much help putting this all together or troubleshooting (it may have been prior to BLFS).

When you said "weirdly useless to understand linux" do you mean the linux kernel in the literal sense? Or the low level system stuff of how things are put together?

I meant the whole linux system, not the kernel. I guess I'm wrong since it's not gnu/linux from scratch. But the two are entangled.