Many students who are doing a semerster abroad will do it in the spring. (Often easier to tack on travel at the end or even additional classes.)
As a result, most campuses have extra beds on campus that term.
For colleges, offering a spring start or first semester abroad is a way of admitting students at the time beds are available. It is both colleges and students who benefit from this.
Also, while it’s less common, there are some students who prefer a spring start. Way back when, I had friends who started in the spring because of fall nationals in their sports.