Trace flags are real helpful when it is used for a specific requirement. Trace flag can set a specific server characteristic or turn off a behavior.
We have Global and Session level trace flag.
There are some recommendations which Microsoft have made for these trace flags:
Global trace flags which are set at server level needs to be set via startup parameter (-T)
Trace can be enabled or disabled using DBCC TRACEON or DBCC TRACEOFF commands
I have already written a topic on DBCC TRACESTATUS which will pull details of which trace is currently active.
I am looking forward to hear from you to understand if you use any special trace flag for your specific requirement.