This blog by Vermaden is probably one of the most cited in the FreeBSD community when someone in a forum/subreddit asks why FreeBSD. I think it does a good job encapsulating the reasons one would choose FreeBSD over Linux or other OS’s.
I’ve become more partial to the *BSD’s in the past couple of years and respect what each one brings. I do daily drive Debian tho ;)
Use case is as I said separating the prefix that is used by your package manager vs. what you are installing your manually compiled stuff in. It helps in avoiding conflicts between your manually installed stuff and the binaries and libraries installed by your package manager.
FreeBSD allows you to compile your own software from souce, which should allow you to choose where you install it…Gentoo’s portage is based off of FreeBSD. Also, pkgsrc is available to you. https://www.pkgsrc.org/