I am new to linux and wants to understand the reason for using some commands. I am trying to install PostgreSQL on CentOs by following the instructions here.

First we do rpm -ivh pgdg-centos91-9.1-4.noarch.rpm which installs the rpm package into the system. But why does it ask us to do yum install postgresql91-server (which i assume installs postgresql) again if we have used rpm -ivh to install the package?

Additionally, what is the reason for adding the lines exclude=postgresql* into /etc/yum.repos.d/CentOS-Base.repo under [base] and [updates]. Why is it that we exclude yum from accessing these packages, then use yum to install postgresql91-server?