Why is the countdown so important while launching a rocket?

A countdown is loudly counting in reverse from 10 to 1, prior to an event. In a rocket launch, a countdown is a carefully devised set of procedures ending with the ignition of a rocket engine, which starts 72 to 96 hours prior to its launch time. During the countdown, step-by-step procedures like placing the vehicle at the launch site, loading the essentials needed for the launch, setting up communication with the launch centre, etc are executed. When everything is okayed, ‘T minus Time’, that is, 3 minutes and 40 seconds is set and the process of ignition begins.