Errorstack tracefiles are very useful for troubleshooting ORA-600's, crashes, hangs and even just bad performance.
Errorstack dumps are dumped automatically by Oracle when critical errors such as ORA-600 happen. This is when you see an error reported in alert.log and the generated tracefile has a "ksedmp: internal or fatal error" string in its beginning, followed by the error code (usually ORA-7445 or ORA-600 with some parameters).
"ksedmp" means Kernel Service Error DuMP, so everything below that line is the errorstack dump.
Errorstack dumps can also be manually invoked by issuing ORADEBUG ERRORSTACK 3 (when being connected to the target process using ORADEBUG SETOSPID). This can be useful when a session seems to be hung (but without showing a reasonable wait event in V$SESSION_WAIT) or is consuming much more resources than normally and you want to know which exact bind variable values are currently used for executing the SQL.