First you can talk to one another about a deep and significanct part of your lives and have real respect there for each others experience and practices.
You can connect not just physically, emotionally, and mentally but also spiritually.
You won’t likely feel like you need to hide or de-emphasize a significant part of who you are.
You can pray and/or do magic together.
The downsides can be like two cooks in the kitchen! You may have different ideas about how to go about something, and you might recall how easy it was to just do it alone. And if you break up you might worry about some magical retaliation. But to me the benefits (cough sex rituals! cough) far outweigh the negatives.