They help a lot when you're dealing with lots of projects where you really care about specific x.y.z release, not just x.y. Then having a common way to manage runtimes is great if you use a few of them.

In short, "asdf install" is way better than download-compile-install python x.y.z, download-compile-install node x.y.z, etc. then making sure the scripts you start actually use the right version.