I'll need to re Britzman's book again (it's heavy going; I had to re-read pretty much every sentence because it didn't make sense the first time!), but I suspect I mis-represented her. I don't think Britzman is suggesting experience has no meaning or value without hindsight, but rather that experience by and of itself does not embody its own meaning. Meaning is assigned to experience by the experiencer. Britzman is challenging what she sees as a myth: that student-teachers lack experience and they will learn how to teach by simply having lots of experience. She sees this as an assumtion that experience carries with it given meaning, because veteran teachers/administrators who express this view often have quite fixed ideas about what it is exactly that they expect student-teachers to "learn" from their "experience". (Here's a Britzman quote that is perhaps relevant to this matter:
The problem was that Jack had borrowed a discourse that was incapable of doing anything other than positioning experience as the ground of knowledge. Such a discourse would not help him with the complications he lived. At this point, all Jack was learning from his experience was that simply being there was no guarantee of pedagogy.
(Update: I agree with Charles that most behaviour is unconscious, and therefore ascribing meaning to experience is also a largely subconscious activity.)
In addition, perhaps Britzman believes or hopes that inviting the student-teachers (and others) to reflect on their experience will help them develop a deeper awareness of what they went through and the various forces at work on them and from within them. I would assume that doing so would stimulate evaluation and re-evaluation of the meaning one had ascribed to one's experience.