DB2 produces diagnostic information which is stored in "db2diag.log" files. Depending on the setting of diaglevel there is more or less information in those log files. Many DBAs have automated tasks to back up and compress and maybe eventually remove those files because they grow over time. On one hand, these files cause some (little) work, on the other hand, they are really handy if something goes wrong. The more information is logged, the faster they grow.
Now let's take a look at one of the many small, but useful new features in DB2 9.7: It's an instance configuration parameter named diagsize. It can be used to configure a max size for the diagnostic information and causes the diagnostic log to become a rotating log. If there is no need to keep old diagnostic information and with a limit set, DB2 will automatically remove the oldest log and recycle the space for the next/newest diagnostic log file.
With the improvement in place, my diagnosis is: Simpler life.