fun fact: robots.txt can also be used by attackers to find admin interfaces or other sensitive tidbits that you don't want search engines to crawl
lots of target-detection crawlers will look at robots.txt as the first thing they do to see if there's any fun pages you don't want the other crawlers to see
If you want to hide admin pages, add the robots meta tag to each one and set noindex, nofollow. Then you don't need to list them all in one place in robots.txt.
That said, obscurity is not really security. Your admin pages should be behind a password, which, if coded properly, will exclude spiders, bots, and bad guys.
lots of target-detection crawlers will look at robots.txt as the first thing they do to see if there's any fun pages you don't want the other crawlers to see