Why are rules in JUnit typically defined as mutable instead of immutable?

I'm new to testing.

I've noticed many rules defined in a test tend to be mutable (var) instead of immutable (val).

Here's an example of testing for Android (what I'm currently learning)

@RunWith(AndroidJUnit4::class)
class MyTestSuite {
    @get:Rule var activityScenarioRule = activityScenarioRule<MyActivity>()

    @Test fun testEvent() {
        val scenario = activityScenarioRule.scenario
    }
}

Why is this? Is there any case where we need to re-assign the rule to something else? Or is it some requirement for JUnit?