This is the sentence I have just read:
外国へ行くとしたら、ただの旅行ではなく、勉強を 目的として 行きたい。 If I ever have a chance to go abroad then I would like to go to study rather than just travel.
The は feels intuitively correct but what purpose does it serve? There are a number of expressions without the は:
～だけでなく…によっても広められている be popularized not only by ~ but also by
～が必要でなくとも although not in need of
あいでなくてなんだろう if it is not love, what is it? (possibly あいではなく、なんだろう？）
I can't explain why は is/is not necessary in these expressions (apart from "these are 決まり文句" but I wanted to get a better understanding than that if possible).
I should be very grateful for any insights.