Deploying a web application or site involves several steps. You typically need to:

Copy files and folders

Provision database schema and optionally populate database content

Set appropriate read/write and security ACLs on files and folders

Install and configure SSL certificates

Deploy other misc dependencies (Event logs, GAC libraries, COM objects, etc)

Performing all of these steps manually is tiresome, slow, and dangerous (since manual deployment steps are error prone).  Enabling an automated process that allows you to quickly provision and deploy applications on a server helps reduce the chances of things going wrong, and can dramatically improve the cycle time it takes for you to get a change you make in an application into production.