What are the benefits of XNU using Mach features compared to a full FreeBSD kernel?

The Darwin kernel is a hybrid of Mach and FreeBSD. What are the benefits of using such a hybrid architecture compared to a full FreeBSD kernel? I'm not looking for subjective opinions of why this architecture was chosen, but am rather interested in technological benefits of the architecture, both from a perspective of today and a perspective of the end of the 1990ies. - Which improvements do the Mach features offer over their FreeBSD counterparts? - What features would be difficult to offer without Mach? Are there things possible with Mach with regards to performance or security that a FreeBSD-only kernel would not allow?