You never know where your router may end up . It may be RMA'ed without proper wiping the configuration first, it may be plain simple stolen. In any of these or other unfortunate cases the last thing you would want is for the attacker get passwords or other security information stored on the router.
One piece of such information is preshared key(s) , that by default are stored in clear text.
To address this potential threat Cisco, starting IOS 12.3, provide AES encryption feature on IOS routers to encrypt the stored preshared keys. In video below I recorded you can see the walkthrough to enable and manage this security feature.
Enjoy. As always suggestions, critics, comments are welcome .
NB - Narration is in English.