Knowing failure means a few things.
First, it means that I know that there are degrees of failure. There is indeed that complete, catastrophic failure - the end - that most people think about when they think of failing. But the actual experience of failure teaches you that it can be far more gentle, and indeed reversible. Having failed, therefore, means many bad things but one redeeming one - overcoming the fear of failing.
Second, the experience of failure also teaches one to plan for failure. This is a departure both from the failure-as-catastrophe view of the world (in which case, no plan is really enough) and also the bravado of not thinking about failure. Indeed, this is not planning to fail, but planning for failure, so that if and when it comes, one has a Plan B in hand.
Third, failing leads to an optimistic view. This does not mean failure is nice - it is never so when you tried something that did not work - but that …