The biggest one for me is that the mental model for maintaining your system is tiny. As a system it is easy to inspect how it works - you can trace the entire boot process along with a finger, the init system is excruciatingly simple, the package database is a directory with files each containing a brief description the output of a un-tar command, SlackBuild packages are easy to make and modify. The community is also very nice and has a lot of highly experienced users in it.

Most software I want is also already installed and ready to go. Some people like to craft their OS from a minimal base, and I absolutely respect that and understand that it gives them a system which is perfectly catered to their needs - but I’m very lazy, and am perfectly happy letting someone with over 30 years experience do all that for me :-)

That being said, I use Fedora at work and wouldn’t consider using Slackware. This is because I don’t want to know anything about my underlying system as long as it works and gives me the tools I need to do my work, and because GNOME is a boon to my workflow.

Over the years I've gone from Ubuntu to Debian to Slackware and back to Debian. I took a chance on Slackware for a few years precisely so that I could force myself to understand *nix from the bottom up. Unfortunately I didn't have nearly as much free time (or patience) as I needed to get the most out of the experience, and having to deal with missing dependencies turned out to be too much overhead for just getting anything else done. I'd definitely recommend it for folks who enjoy (and have the time for) getting their hands dirty.

Slackware is more bare bones, so its good for learning linux.

It is also stable, people set up slackware servers and let it run for years.

But to be honest, modern distros are much more convenient esp for workstations. These days Slackware wouldn't be my first choice, but it brings up memories, had a lot of fun tinkering with it.

It is very old-school. A more extreme choice in the same vein might be Linux From Scratch.