Auto versus Lock hubs ????

Hello, I have a 2011 F250 LWB 6.7 Diesel XLT 4x4. I have a question about the auto locking manual hubs. I read the owners manual and can not get a clear idea about the hubs. There are to features on the hub , one says auto or you can turn it and put them in lock position. My question is if they are in auto all the time so that you can shift on the fly with the dash control why would there be a need to lock them in ? I guess I'm missing something ?

I use the auto hub lock if I run into ice and snow in the mountains and dont want to get cold locking the hubs in or at the boat ramp for a short pull out. But if I'm running for some time on muddy roads then I'll lock it in at the hubs. Sometimes the auto hubs dont lock in right. As one hub will one wont.

The auto hubs are vacuum actuated and lock when the dash switch is used. The manual lock position is there because the vacuum system sucks and has a habit of leaking. Of course you won't know about the leak until the hubs didn't lock when you thought they were locked.

Here is what I was told by a Ford engineer on another site . He said the shift on the fly (auto pos.) is for quik activation of the 4wd system . Like when you are possibly not expecting to need it and suddenly you do . Like said before just driving along and suddenly hitting a slippery section of road . The lock position is for when you know you are going to need the 4x4 and / or are planning to have 4x4 engaged for an extended period of time . You still have to flip the dash switch to engage the transfer case .

As far as because of the vacuum seals leaking , if you do regular hub mait. you shouldn't ever need it for that reason . That being said if you should experience a vacuum failure you can get out and lock the hubs and still have 4x4 .

Hope this helps