Everyone knows that 80-years-ago you could get a 'good' job with only a high school education, while you now need some level of higher-education. This isn't because young people are (necessarily) gaining important life/job-related skills during these four years after high school. It is social signaling, or - if you like - credentials inflation. The thinking is: if you can make it through a bachelor's degree then you must be reasonably intelligent to do entry-level work. This says nothing about the actual need for higher education.
By de-coupling educational attainment from age offers the opportunity to advance young people through stages of education based on their skills, not merely their age. In doing so, graduating from high school becomes a standard moment for progress benchmarking. Some students will show significant ability, others won't. Advanced students will go on to do advanced work and lower skilled students won't be penalized (as they currently are) with having their high school diploma withheld from them (a significant employment problem for many low-skilled workers).
It allows businesses to look at high school graduates and assess whether or not their skills are adequate for the current opening rather than merely assuming that a bachelor's degree is necessary to do the work. In this way, we incentivize trying to advance quickly through high school while also de-incentivizing the default choice of a four-year degree.