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.

If you're coming from a linux environment, you're probably used to having Apt or Yum available. OSX has no built in package manager so you've either got go out and find compiled binaries somewhere or install from source, both of which are tedious and searching the web for it is time consuming vs. the `brew install wget` commands that you can probably guess and it'll take care of installation in the background while you do something else.

See here for stuff you can install with one command: https://github.com/mxcl/homebrew/tree/master/Library/Formula...