C-51 gives law enforcement and intelligence some slightly more modern tools. For instance, they can demand that "terroristic" material is removed from the internet (where jurisdiction applies), which is a right that they already have for child pornography and hate material, with the agreement of a judge and the minister of justice. If they become aware of a subject, they can ask that the subject's ISPs capture data pending a warrant (versus waiting for the process of the warrant and perhaps miss things).
And so on. Remarkably few who comment on C-51 have any idea what it entails, or the scope changes it offers. But they ply, ironically, exactly the sort of rhetoric of fear that the right wing plies about terrorism -- somehow having an advance ability to request data capture == All lefties are going to be locked up.