Pacman encompasses installation, configuration and setup in a consistent system. For example, in order to use software one normally has to add locations to paths or define certain environment variables or perhaps source some shell scripts. The problem is that if you need to use several pieces of software the setup procedures might interfere with each other. For instance, two packages might put two versions of the same binary in your path causing a problem for either one package or the other.
Pacman lets you solve these problems in a unified way by using Pacman for setup as well as installation and configuration. For example, if packages PackA, PackB and PackC, are already installed on your system in Pacman installations at /home/a, /home/b, home/c respectively, you can set yourself up to use all three together by just doing
% pacman -get /home/a:PackA /home/b:PackB /home/c:PackCPacman then checks that these three packages are consistent and, if so, it generates setup scripts for you. If they are inconsistent, Pacman tells you this using the same familiar mechanisms used for installation.