It's preference. For me, locked limits your visibility. Since you don't understand what people are saying, I made it simple for you and have pictures! yaaay
Picture number 1, Locked View:
With locked view you are restricted from what you can see further in your lane and you have excess, unnecessary space behind your champion.
Picture number 2, Unlocked View:
The glories of having an unlocked screen gives you flexability. Without moving my champion, I can now see further down my lane and into the top bushes.
Picture number 3, Unlocked View:
With little effort, I can now see more down the lane.
Picture number 4, Unlocked View:
And now I can view the river and its bush.
Think about it like driving with mirrors. You don't always stare out your rear-view mirror, do you? No, you check your mirrors at the side of your car too