Technically speaking, OpenRC doesn't really have any benefits in the real world, some people may claim faster boot times, but that's debatable on modern hardware. In fact objectively, it's inferior to systemd in many ways.

The real advantage though is that it's pretty simple and easy to use, understand and maintain. It follows the Unix philosophy of "do one thing, and do it right". People who like to have full understanding and fine control over their systems would prefer using OpenRC or similar init systems (with a mix-and-match of other utilities and daemons as per their need), instead of relying on a giant monolothic package like systemd which keeps getting bloated with more and more "unnecessary" features with each release.

Basically, you can say that it's a difference of ideology.