Girls seem to generally like the experience, recommend it to each other, and this might be the first question they ask each other when introduced. That is seminaries achieved the goal of making themselves the default option.
there are multiple questions here, not just one:
1) what are they to do anyway after HS? Unless you follow gemora that presumes girls mostly married after bas mitzva, you have an answer. Otherwise, she would be learning, working, or going to college.
a) Learning – in this case, doing it in Israel is a reasonable choice, especially for those who are going in into chinuch later. In Bava Basra discussion of schools (for sons), going to Yerushalaim is a second choice that was made after being taught at home: enthusiastic teachers in holy environment
b) Working – post-HS jobs are not always attractive. To work in a school, having 1-2 years of seminary would be helpful, although not recoup the pay, of course.
c) College – for those who send kids to a college away from home, a year of seminary might help to strengthen them spiritually. Also, many provide college credits that are at least acceptable by a couple of Jewish colleges that provide safe spiritual environment at a price (price of a college with better academics).
d) Safe college – a commuter or online college, and there is parental supervision and local learning and socializing options. [There is a 20-yo old article in some OU magazine by a campus Rabbi (sic!) telling people to send kids to local colleges, instead of “American tradition” of sending them away].
In this case, the seminary is not so useful and may be also not conducive to the girl’s development (exposing to additional teachers with some ideology) and possibly not able to use those credits when used in local/online colleges.