We see that Erik is given the helmet by Apocalypse- this links the Magneto persona to both Apocalypse and Shaw (the villains are all connected as Heidi pointed out a while back) because Erik gets the helmet from both of them.
This is most likely done to protect him from Charles’ interception and influence and probably Jean too a bit. Erik looks so sad when he takes the helmet because I believe he doesn’t really want to cut off Charles but he feels he has to.
However, what if it is how he starts to see through Apocalypse? That helmet basically makes him immune to all mental manipulation of any kind. Psylocke has telepathy in canon, but I’m not sure that it will actually be used for the movie version of her but if it is, his helmet cancels her out.
However, as is shown in X2 one of the things that Magneto’s helmet protects him from is reality warping, which Heidi and I think will play a part in his story arc.
The part of the movie where Apocalypse walks Erik through his traumatic past and loss (that we are assuming happens) would happen before he’s given the helmet. However once he’s got it and meets Peter (Quicksilver), or in someway comes across the idea that that is his child, his family would not all be dead, and Apocalypse can no longer use that to keep him as a horseman.
Obviously this is all speculation but if this were the case it would echo Singer’s repeated stating of how this movie fits within the canon of all six movies, plays homage to them, while also being the birth of a new direction. You have this helmet which has kept Erik from being with Charles (and their family), both physically and mentally, for all these movies, and becomes the step that begins his journey home.