1) Provides a chance to get expert feedback on work-in-progress manuscripts and;
2) Provides the only chance many of us get for in-person interaction with colleagues working on similar problems (because many academic departments are staffed to cover all disciplinary bases, not in-depth in any given subfield).
I'm not personally that bothered about sabbatical time/research leave, but if you're in a discipline that writes books rather than papers it's very helpful to have an extended period to sit down and write. And for those who strongly prefer research to teaching then a period of research leave gives relief from the grind of what's seen as a 'day job' and the opportunity to work on research with your full brain. Some people use it similarly to 20% time to work on something novel that might be important.