Some trace flags are used to enable enhanced debugging features such as additional logging, memory dumps etc. and are used only when you are working with Microsoft Support to provide additional data for troubleshooting. These trace flags are not ones you want to leave turned on in a production system as they may have a negative impact on your workload. An example of one of these flags would be TF 2551 which is used to trigger a filtered memory dump whenever there is an exception or assertion in the SQL Server process. These trace flags are only used for a short period of time and typically only at the recommendation of Microsoft Support, so they will likely always be around.
If you are a DBA and are not extremely familiar with trace flags, you really want to read this article.