When we are working in a farm of SharePoint with more than 1 servers and each server was configured to run different roles or services, the logs are generated in the different servers depending the role or service configured. This is a problem when we need do a research about an error. For this reason SharePoint has a command that will be using to merge all logs in one file, this command is Merge_SPFileLog.
How to works, get access via RDP to one of the server of the farm, run as administrator the PowerShell console of SharePoint, execute the command using the correct parameters for you. In the following pic you can see the execution of the command.