The struts.devMode is used to make sure that framework is running in development mode or production mode by setting true or false. struts.devMode is set to false in production PHASE to reduce impact of performance. By default it is "false". 
It is used because of the following reasons:
Resource Reloading: Resource BUNDLE reload on every request
Modification: struts.xml can be MODIFIED without restarting or redeploying the APPLICATION
ERROR Handling: The error occurs in the application will be reported, as oppose to production mode.

The struts.devMode is used to make sure that framework is running in development mode or production mode by setting true or false. struts.devMode is set to false in production phase to reduce impact of performance. By default it is "false". 
It is used because of the following reasons:
Resource Reloading: Resource bundle reload on every request
Modification: struts.xml can be modified without restarting or redeploying the application
Error Handling: The error occurs in the application will be reported, as oppose to production mode.