Biphosphonates prevent skeletal events fractures, osteoporotic complications, and address bone pain with metastasis to bone. That much is certain.

When I was treated in 2015 all the docs said that Reclast (the specific one) might help prevent spread, by affecting osteoclasts. Studies were mixed, but here is one done in 2014 including Reclast, that found protection against recurrence for those who took Reclast with an AI right after surgery, who were hormone positive, and post-menopause. (These were the folks in the study.)

I found one study that said Zometa plus Femara reduced risk of recurrence by 30%. I also read a study some time ago that said all cancer was eliminated with the same combo.
Here is the first one mentioned: https://www.breastcancer.org/research-news/20140128

I honestly think they don't really know but there are signs pointing to that effect. The mechanism would be by stopping bone turnover via osteoblasts (which worries me on Tymlos, and Tymlos is not a good idea if you have bone metastases!).

I like to write precisely and would add "may" have protective effect. But I would also clarify they definitely have a protective effect on bones with cancer- better wording is "with" not "for" unless some of those studies are on target.

I think anyone considering this med should ask their oncologist for the latest research on this.