I do think that it is necessary because many times that is the last year that they will receive a full dose of hashkafa for the rest of their lives, unless they make a strong effort to seek it out, and at 17/18 I dont think anyone has all their hashkafos set for life.
I’m not sure however how necessary going to Eretz Yisroel is. Yes, it is very nice to see the kedusha of E’Y, and to see how people live “gashmius free”, but meanwhile at home the parents are tightening their belts so that they can spend the extra 20k. Bottom line is I dont think EVERYONE gains, there are definitely cases where it has done a world of good for girls (and unfortunately the opposite is true) and then there are the majority of cases where the girl just had a year in E’Y where the focus was the teyulim and the fun… for 20k I can say Ma Rabu Maasechu Hashem at the Grand Canyon, it doesnt have to be in E’Y, and it would be a whole lot cheaper.
Boys going to E’Y to learn is a completely diff story, first of all the money issue is non-comparable, it is much cheaper for boys. Also, there is an inyan that you learn Torah much better in E’Y because of the Kedush (please no one say that girls are also learning Torah, I may lose my lunch)