- Looking at which modules had the most defects in the past can tell you to look at those modules early, because they're likely to produce defects.
- Looking at which modules had a lot of defects in older releases, and then fewer defects over new releases can indicate that the module had underlying issues which are now fixed. Translation: spend a bit less time here.
- Looking at where you find issues in the field, and which customers have problems. What about their use cases are we not accounting for properly?
- Looking at where in a release issues get introduced: during development, right after feature completion, late in the test cycle, etc. Moving defect discovery up might become a goal, if you're finding bugs too late.
As always, research is better with a question in mind, but consider researching in your defect tracking system. You never know what you'll find!