There are 2 (main) reasons that speaking Norwegian in Norway is important.
Firstly, there is a misconception that "Norwegians are good in English". That is quite a broad statement, and unfortunately it's not quite true. It's more accurate to say that "some" Norwegians are good in English and many Norwegians understand a fair amount of English.
Norwegians on the whole are not exactly brimming with self-confidence, I don't say that in a condescending manner, but that's just the way things are. It's because of a moral law that has been ingrained in society for many years, that prevents people from being recognized as successful and indeed even talking about being successful (in anything) is not the done thing. (this is improving a little but on the whole it is still the norm).
So, in conclusion. Those who 'are' proficient in English don't 'think' they are, because no one ever gave them credit for it, because giving others credit, is not the done thing. And those who are not proficient in English, because they haven't used it for years, or didn't pay attention at school, don't like using it unless they absolutely have to, and a foreigner needing a job doesn't constitute having to, so the burden is on the foreigner to learn it. (if you want to at least be considered for a job).
The second reason is nationalistic. Norwegians are, on the whole very proud, and love Norway, and if you want to be a part of it, then you have to show that you feel the same way by learning the language and partaking in various things like voluntary community work etc.
English is used a lot in Norway these days, but more in some parts of the Country than others. It took me a long time to get a job because it took a long time to learn Norwegian. The best tip I could give someone learning Norwegian is to realize that 85% of it is pronunciation. You have to get that right and it only comes with practice and getting constantly picked up on it by friends who are close enough to be honest and tell you.
As a foreigner you get the impression that "Norwegian" is simply Bokmål (or Nynorsk) or simply put, "school book language", and that's it, but wait there's more....in Norway there is a mind blowing amount of dialects, and in many cases they can sound like 2 different languages altogether. Take for instance the Stavanger dialect and one of the Trondheim / Trøndelag dialects. if you learn, and become reasonably proficient in either one of them, your ear will be trained to the different sounds, but move from one to the other and you'll feel like a foreigner all over again.
The sickening, but amusing part about this, in my case is that the job I finally got, was English speaking.