The wrapper.log file is a file on the file system where Ignition is installed.
Ignition logs errors and various important information about what Ignition is doing to the wrapper.log file.
The wrapper.log file is used to test and debug Python scripts that execute in the Ignition Gateway. Gateway Event Scripts, Tag Event Scripts, Python scripts in Alarm Notification Pipelines and Python scripts in Sequential Function Charts all output print statements and logging statements to the wrapper.log file, and the wrapper.log file can be used to debug or test any of them.
The wrapper.log file also collects a lot of information about the internal workings of Ignition. If something is wrong with Ignition the wrapper.log file is a place to go to see if some error is being thrown causing things to not work correctly. If you want to know what and when something is happening in the Ignition Gateway the wrapper.log file may have some useful information about it.
Ignition is very often installed on a computer server that most people do not have local access to -- meaning that people have to gain remote access to the computer in order to access files on the computer. Since the wrapper.log file is stored on the hard drive of the computer where Ignition is installed people have to use a remote access tool like RDP for windows or SSH for Linux or something like VNC. This also means these people need a username/password to log in to the server. This can be a lot of trouble just to view the wrapper.log file.
Some developers may not have or want remote access to the server where Ignition is installed, but if they don't have it then they can't view the wrapper.log file. This problem is solved to a degree by having administrative/configuration access to Ignition's System Console webpage where much of the same information from the wrapper.log file is displayed. But again, it requires that users have access to the configuration of Ignition's gateway in order to see logging information. Developers may not have administrative access to the Ignition gateway. And system administrators may not want to give administrative access to Ignition to all Ignition developers.
The Wrapper.log Viewer project solves the problem by providing easy access to the wrapper.log file in an Ignition client or designer.