What are the advantages of a multi-site installation?
CiviCRM multisite allows you to have several organizations sharing the same instance of CiviCRM, but only a superuser can see the data across all sites.

The most common example is an organization with multiple chapters.  You mention your organization is active in different countries.  If:


You require that users from country A not be able to see contacts entered in country B;
Users in a "head office" need to see contacts from multiple countries in a single place;


Then multisite is for you.