I think the most important work for a manager is to :
a) Find, Recognize and Hire talented people.
b) Make sure that the talented people figure out how to work together.
c) Improve and optimize the processes and the organisation ( continuously and in small steps.)
d) Arbitrate discussions and help making decisions, but do not take them on your own
e) Especially in larger organisations, evangelise about skills and every good thing that has been done by your teams.
f) Have an eye on the horizon now and then. Engage the teams in strategic discussions and long term planning.
To do these things well a deep knowledge about software development is required. ( Or about teaching, or medicine, or whatever it is the organisation is doing.)
It's not possible to get this sort of insight without having practiced the trade for some time. Yes, it possible to manage without, but then there is a high risk that things go wrong in some - and then maybe all - of the above areas, simply because it is easy to misunderstand some things and fail to recognise others.
Another risk is that the important things are replaced with less important things:
v) Make sure that everyone is aware of deadlines, project plans, priorities.
x) Order stuff that is needed.
y) Make budgets, and report progress.
z) ...or even : Handle and approve vacation requests
Sure, these things must be done, but it isn't exactly rocket science and everyone and their dog is capable of handling these tasks.
Less knowledgeable managers and project managers tend to focus a lot on status reports and reminding of deadlines,
sadly adding about as much value as an automated mail could have done (I'm looking at YOU tick-box-guys) while missing the important stuff.
One problem with non-technical managers is that they may 'accidentally' accept unfortunate (technological) decisions made outside the team without challenging them, or even worse make their own, perhaps because they fail to see the implications. They will then end up defending senseless decisions or policies against the team, generally having to revert to "just because" arguments, and since the decision may not be easy to back from once committed, everyone involved will become angry or whiny and the team will become generally obstructive and unhappy.