If you’re a publisher with content behind a login page, be sure to set up an AdSense crawler login so that your ads are as well-targeted as possible. Doing so will give the AdSense crawler the ability to scan content on these pages, resulting in more relevant, targeted ads that can potentially increase your CTRs and revenue. Please note that adding a crawler login will only help to deliver better, more relevant ads to pages with AdSense code already on them, and will not have any effect on which pages of your site Google Search will index.
To grant the AdSense crawler access, log in to your AdSense account and click on ‘Account settings.’ From there, scroll down to the ‘Access and authorization’ section and click on ‘edit logins’ next to crawler logins.
There is nothing Google Adsense would want behind the login, but it has been asking me for that for a while. Everything is accessible, and the login allows others to post comments and similar activities.
I hope Google doesn't think we are hiding something behind the login.