The Start Button is located near the bottom of the Account Window. It's purpose is to trigger the process of logging into, and/or loading the settings for, the account that is currently displayed in the Account Selector. Whether that process involves connecting to the DAZ 3D servers or not depends on whether the Work Offline Option is checked.
When working_online, if authentication fails for some reason, like the account credentials do not match what is known by the store or there are connection issues between your computer and the store, a message will be displayed to alert you to the situation. If your credentials are correct and a connection is established, but you have not agreed to the current End User License Agreement (EULA), you will be prompted to do so. If a newer version of Install Manager exists than the one you are currently using, you will be prompted to update to that version or switch to working_offline mode.
Below is a list of interface elements that the Start Button can exist within: