It is an American idiom meaning whatever "it"is, is a great benefit to the situation. i.e. if you're in an accident, it i a big plus to have good health insurance.
I'm wondering the same thing - does it mean 'a positive' in this context?
Why isn't great accepted here? Stort means great, and great and big are interchangeable in English.
Great and big can be equivalent in situations in English, but they are not inherently synonymous. We treat great as a translation of e.g. jättestor in Swedish, which is always larger than stor. So if we were to consider great and big to be always interchangeable, it'd be confusing to learn how to translate them.