There is some amount of new players who find it easier, so it's not unexpected that you do. It's one less thing that you need to pay attention to when LoL has tons of things to pay attention to.
The reason why unlocked is better is because it gives you more vision of what's happening. When the camera is centered on your champion, it most always does not give you vision of what you need to see, like lane entrances, and less vision of what you are looking at. Most of the time, a large amount of your onscreen field of view is "wasted" on the camera showing what is behind you when there is no need to see there. I, on occasion, switch to locked just to see the difference and I can't stand it because of how little I can see. It further hinders aiming with champions who are long-ranged, especially as there are champions whose abilities are even longer ranged than what the locked screen will show like Xerath, Ezreal, Twisted Fate, and Ziggs
Here is a video that demonstrates what I am talking about visually.
LOL Keyboard Mechanics Explained Camera part starts at 5:22, but you might want to watch the whole thing if you don't know about self-cast and such.