It’s very convenient to use ScriptApp.getOAuthToken() in an addon to reuse the token from the server side in your client side add-on code. However, these have a limited time to live (1 hour), so what to do if your add on sits around for longer than that? One solution would be to always go for another token every time you needed one, but that would be wasteful. Luckily, there’s a way to check a token to see how much life it has, and only get a new one if it’s almost expired.
There is also a related discussion thread on this post with the author, Bruce Mcpherson, in the Google Apps Script Google Group.