Generally any scripting or programming languages will provide a option to import the same logic source code from other application or pre-existing utilities. In the same manner Qlikview scripting language provide a option to developers to import the external qlikview script files into qvw file. By using "Include" function in the script we can import any file have qlikview script code.
This functionality is more useful to change the DB connections if we deploy the qlikview application from Dev to QA or QA to Prod. Simply change the DB connection string on the scripts files so we don't need to do any script change while deploying from QA to Prod etc.
The one draw back is syntax checker didn't highlight any script error while validating. so developers needs to take care of correct syntax on the script files.
The other drawback is, if script file doesn't exist Qlikview script execution didn't throw an error. To avoid this we can use "Must_Include" function instead of "Include"