When installing mysql in linux, why do we create a mysql group and user?
Every process under linux runs under specific user privileges. Services (like MySQL) usually need to open ports and access various system resources during startup, so they are required to be started as root user. However, it is not safe to have all the processes run under root as it is not required for continuous operation of services, thus it is recommended to create a special user, which will be used to run MySQL service. MySQL will only be able to access what special user can, and this is going to be limited to MySQL files on the system. 

This is usual practice in linux. If you, however, use you distributions built-in package manager to install MySQL, this will be done for you automatically (in most distributions at least).