What is the difference between “important for” and “important in”?

Question

• The Independent: maintaining independence is important in relationships.

• The Telegraph: Protein intake is also very important in building muscle.

• BBC: Meals are also important for building business relationships.

• The Guardian: Personalised content is increasingly important for building brand relationships.

• The Guardian: She says the course is especially important for building the confidence of teenagers who lack support at home.

I think there is a difference in meaning between "important in" and "important for", but I don't know what it is.

Answer 1

The meaning of important is the same in all of these sentences. The choice of in vs. for is governed by the words that follow. The examples with in use that preposition in the sense of "part of" or "an ingredient of", and the examples with for use it in the sense of "in order to", i.e. to say that something is a prerequisite for another thing.

You could rewrite these sentences like:

Maintaining independence is an important ingredient of/plays an important role in relationships.

Protein intake is also very important as part of building muscle.

Personalised content is increasingly important in order to build brand relationships.

or, Personalised content is an increasingly important prerequisite for building brand relationships.

In some of these examples, both in and for would work - for example, protein intake is an important part of building muscle, but it's also a prerequisite for building muscle, so you could say "in building muscle" and "for building muscle".