Examples:
1. your exception messages contain the id of the entity you're dealing with "FooBar 1234 has broken". You only want to match then on the FooBar has broken bit, not the 1234.
2. for whatever reason, some IP address keeps sending you dodgy requests that break your server. You can filter all errors requests from this IP address so you don't get swamped with these errors.
3. The same bug deep down in your code is causing different errors higher up. You can choose to match on the part of your stack trace that is the common cause and get all these errors attached to the same issue.