https://admissions.berkeley.edu/make-most-january-start describes UCB January start / spring admission. However, the Global Edge and Fall Program for Freshmen fall options for these students have relatively limited course offerings and may not be that suitable for students intending some majors.
One likely reason (for any school that does this) is that starting some students in the spring balances enrollment across the fall and spring semesters, since fall semester would ordinarily be more heavily enrolled because students graduating a semester early or late would have an extra fall semester. But ranking-conscious colleges may also use it to try to game the ranking systems that count only fall frosh in selectivity stats.