In addition to helping users create applications, GeneXus always strives to boost their productivity. It is common knowledge that as projects grow, you need to move on to using more than just the Developer Menu.
The aim of the Startup Object is to allow you to specify which object will start running when F5 is pressed so that it is "set" in the Environment.
The object specified as the Startup Object will be the one that starts the runtime session until another object is set as Startup Object.
For more information read Setting an Object as Startup Object and Unsetting a Startup Object.