In an "ideal" agile environment a feature goes from User Story, to Use Case, to implementation. With tests along the way, you should be fairly confident that by the time you mark a story done that it meets the technical requirements you were working from. If someone is testing your application after this point, and the developer tests are good, it seems like they should be testing something other than the code, which has already been tested.
Even though it's important that your QA team does not become the place where "testing is done" as that will sidetrack an agile project very quickly, it is possible that the QA Team is testing (ideally in collaboration with developers) things that the team decided that developers could not test completely. Also, in practice, problems sometimes slip through developer tests, and the QA team is then testing the developer tests to give feedback on the types of things the developer tests need to be better at.
Aside from "catching the errors that slipped through" a QA team doing exploratory testing is also exploring novel paths through the application, and may discover problems during some of these paths. In this case the team and stakeholders need to have a conversation about whether the error is something that users will see and care about, or whether it is something that isn't worth fixing.
In effect, exploratory testing is testing the requirements for completeness.
If you have a QA team, and they are doing exploratory testing, they are really testing:
- Requirements: Finding interactions between features and components that were not defined or understood when coding and developer testing began.
- Developer Tests: Identifying where developer tests were not as good as they could have been (and how they could be better). This is a good use of automated "integration" tests.
- Systems tests that might be hard to test in a developer context.This might be the one set of tests that are the primary domain of QA.
Of course, this is an idealization. But if you are disciplined about doing developer testing, you can still get a lot of value from exploratory testing, whether the people doing them are dedicated QA people, or people who are filling the role.