Speed and reliability. Take open source. You could manually find, download, untar, configure, make, install and track dependencies. A good package manager lets you do that in one command e.g.:
$ brew install package_name
OSX includes a package manager for proprietary packages blessed by Apple. It's fast and reliable but its curators forbid most open source and proprietary packages. It's called App Store.
Ignoring the problem of software availability, the App Store also lacks a fast command line interface. I can kick off a homebrew package install process and return to whatever I was doing faster than the App Store.app launches.
