In Days of Future Past, Wolverine's consciousness is sent to the 1970s to warn a young Professor X about the threat that the Sentinel program poses. In order to prevent the Sentinel program from being developed, they have to prevent the pivotal event that kickstarted it (Mistique's assassination of Dr. Trask) from taking place. But wait! Wolverine, Professor X, and Beast alone, badasses though they are, cannot pull this off alone. They need Magneto's help, and that involves breaking him out of a super-hyper-ultra-high-security jail. The daring extraction plan having succeeded, the heroes travel to Paris and manage to foil the assassination attempt at the last second.
Except that, when you look at it, Magneto didn't actually do anything up to this point. He went along for the ride, but the one who orchestrated everything (including telling Mistique not to pull the trigger) was Professor X. I can see the need for Magneto and Professor X reconciling, but this is something that could have been relegated as a not-so-urgent task, to be dealt with when Trask was discredited and the Sentinel program cancelled for good.
So, what exactly did Professor X need Magneto for at this point in the movie?