multiple site installation vs single site installation

New to Wordpress. I would like to know if it is advisable to install an instance of Wordpress on each secondary domain on my server. At first a copied all the Wordpress files to each secondary domain folder. My hosting provider suggested that I install Wordpress on each secondary domain instead of coping the files. Unfortunately after following the instructions my Wordpress sites gave me errors. Should i be installing an instance of Wordpress on each secondary domain or is one instance of Wordpress installation sufficient. I also have an installation on the primary domain. Thanks guys