> "act as an umbrella", really means protect you from other managers
Eh, not only. Other non-management employees can mess up your time quite effectively, as can people you're working with outside your company (clients, vendors). Sure you can shield yourself from it if you try... or you could exploit division of labour and get someone to do it for you so you can concentrate on what you do best.
Experienced well-rounded employees can mostly take care of themselves (and know when to say "no"). It's the new-hires and rough-around-the-edges people that require more hand holding and can benefit more greatly from a manager.
Yes, they can. But that distracts us from our core job: writing code.
A good manager can get others off your back much quicker and shield you from the political fallout that can occur - essentially being the bad guy for you.
Eh, not only. Other non-management employees can mess up your time quite effectively, as can people you're working with outside your company (clients, vendors). Sure you can shield yourself from it if you try... or you could exploit division of labour and get someone to do it for you so you can concentrate on what you do best.