[Grammar] "gains in importance" or "gains importance"??

[pinbong]

Hi, teachers:

The following sentence comes from a piece of article I've mentioned before in another thread of mine: https://www.usingenglish.com/forum/...press-reason-what-does-mean-2.html#post692551

But in dealing with the young, the fact of association itself as an immediate human fact, gains in importance.

I've read someone wrote "this new project gains momentum" (without a "in" inbetween)

Since "gains in importance" was written by Mr. John Dewey, and "gains momentum" by a top journalist, I suppose both usages are grammatically right, aren't they?

What's the subtle differences of using "in" or not? I can sort of sense it but can not clearly express it.

 

[Tdol]

gains momentum = acquires momentum- could be from stationary
gains in importance = increases importance- already had some importance at the start