Why I have to promote by the package?

Package ID promotion - is a good option for applications that need a large number of installs. Primary it suits new application that were recently located on the market and are even not seen for the users who are in search.
Package ID promotion will help you to lead your application to the visible parts of the market and open new opportunities for your application, such as keyword promotion (read here).

However, buying package installs via direct country you are growing up in countries apps' rank. Also, after package ID promotion you could start another way of promotion - keyword promotion (read here about keyword promotion).