Thanks steph_giles - so in LastPass this is the default security setting for individual logins. Even if I'm signed into the vault, if I want to view the full-text of the password, I would need to re key-in the master password to show/reveal it. Lastpass does give the option to turn that off (i.e., "don't ask for master password after 15 minutes/30minutes/ 1 hour" etc.)
I suppose the use case for this would be if I were on a shared computer or somewhere else where someone would be likely to be looking over my shoulder. I guess if I were very worried about this, I would set the timeout timer all the way down to 1.