They aren't doing it to hide anything (outside of perhaps those in with a chance of World firsts).
They're doing it to stop people going for ranks on progression. If you have public logs, someone is always going to try and cheese or whore, even if it's just a little bit here or a little bit there.
My guild opted for private logs on progression and for a few weeks after clearing a tier to ensure smooth farm runs and focused progression. Other guilds may have other reasons but this was ours.