Multiple versions of the same package can be handy: e.g., Python 3.6, 3.8. and 3.10 all handled by the package manager and able to switch between them on a per-

Multiple versions of the same package can be handy: e.g., Python 3.6, 3.8. and 3.10 all handled by the package manager and able to switch between them on a per-session/terminal basis (no need to completely disable a particular install: just adjust $PATH and $LIB as needed). Able to create pyenv-like environments which can then be 'exported' and sent to other systems/users having Spack so that development/debugging can be done with the same setup: * https://spack-tutorial.readthedocs.io/en/latest/tutorial_environments.html https://spack-tutorial.readthedocs.io/en/latest/tutorial_env... Able to use 'internal' packages (spack install openssl) or external packages (link against /usr/lib/libssl.so) on a per item basis: sometimes you want the newest code, but other times you want the OS package so that you can get security updates 'for free'. Most other system do one or the other: Spack allows you to choose.