If your boss can do your job, he can understand what you're doing and correctly determine how you perform. One of you is only redundant if the manager doesn't have his time filled with other things.
An engineering manager actually reads the output of the engineers and participates in having it applied to realise bigger projects. He makes higher level decisions about the engineering projects.
You don't really want your senior and staff engineers bogged down with career management, hr disciplinary implementation, team salary budgeting, and all the miscellaneous other duties of a manager. You want them thinking deeply about the code, the business problems, and how the two meet.
In terms of evaluating job performance, you've got peer reviews, deadlines met, contributions in meetings and so forth.
In other words, a manager should have their time filled with other things. Unless the team is really tiny (too many managers), there's plenty for the manager to be doing- planning with product / sales / design teams alone should be enough to keep them busy.
I think that's the crux though, for some reason companies think that a team of 3-5 engineers needs a full time people manager. Unless you are dealing with all juniors or a bunch of primadonnas I don't see how every team needs their own people manager.
An engineering manager actually reads the output of the engineers and participates in having it applied to realise bigger projects. He makes higher level decisions about the engineering projects.