If you received a retraining token in a pack and don't want it to take up space in your bank or inventory (I've now received about 5 !) as long as you're happy to commit it to a particular toon, you can activate it - its then available for your toon when you're ready but you don't have to use it right away.
You can test out your new build on the Mimic or Owlbear (test) server to see if your planned build changes turn out better than your current build, without wasting your precious retraining token.