It stimulates the bone tissue to grow more dense. Resistance training puts additional stress on the entire body -- our bones aren't just stagnate pieces that we grow once and they forever are the same. They are living tissue and are constantly regenerating themselves.
With sedentary lifestyles (common today) the tissue becomes weak because it isn't stimulated under load. Much like getting 'out of shape' or losing muscle mass, strength, size. The muscles respond by stimulation (resistance) and thus adapt to compensate. Our bone tissue operates under a similar principle.
In fact with Calcium supplementation, Vitamin D (supplementation and in some cases increasing direct sunlight per week), with resistance training can *reverse* some osteoporosis?
How, well.. bones that are ostenopic have become somewhat brittle (of course there are many varying levels of this degeneration..) -- but at any rate they become thinner, less dense, and generally less structurally sound. When you add the nutrients and just as importantly the increasing load that wasn't there before (resistance) they *over time* will react and grow more dense, stronger, and healthier.
This leads to overall better health, strength,.. and a MUCH less likely chance to fracture as a result of osteoporosis.