If your boss can do your job, one of you is redundant.
Managers of engineers should be the interface to the business and other units within the organization. That's a big enough job without requiring coding skills on top of it.
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.
I used to work for this guy. He was a network engineer by trade, and I understand a very good one.
However, his story goes, one day he was asked to design a telephony solution for a tech support call centre.
He fell in love with the call centre. He very quickly gained a deep understanding of what they were trying to achieve with the telephony solution and pulled a bunch of critical data out into a dashboard for them after learning how they operated.
He asked for a job and they gave him one. He was an effective manager because he had a deep understanding of the role of the 200 staff he managed, and was able to effectively spot issues with staff based on their metrics.
He could have done my job, but understanding it gave him the exceptional ability to add value to 200 staff at once. Neither of us was redundant.
Managers of engineers should be the interface to the business and other units within the organization. That's a big enough job without requiring coding skills on top of it.