Canon says it's supposed to protect the AF when traveling.
How does locking the zoom barrel from extending protect the AF?
I'm not sure, and will have to look for the source, but I know I read a CPN article or something where they say that the locking mechanism was designed to protect the lens while in transit. Most people assumed it was to lock it closed, but apparently that's not the case. I can't seem to find it at the moment but will keep looking...
EOS 1D X, 5DIII, M + EF 24 f/1.4II, 50 f/1.2, 85 f/1.2II, 300 f/2.8 IS II || 16-35 f/4 IS, 24-70 f/2.8II, 70-200 f/2.8II || TS-E 17 f/4, 24 f/3.5II || M 22 f/2, 18-55 f/3.5-5.6 IS || 1.4x III, 2x III