By making an embryo transgenic, we've successfully inserted DNA into the somatic cells (the ones that make your body, as opposed to the gametic ones, which make the sperm and eggs). This means the new gene is present in all of the cells of the body. While it's been done in rats and mice, it's not been done in a higher mammal, so putting it into canines is the next step.

How does this have applications for fertility and disease prevention? Certain diseases are caused by genetic deficiencies (think MS and the like). If we know that this is likely to be a problem for the baby, we could theoritically insert said proper and uncompromised gene into the developing embryo and prevent the disease from ever occuring. For some diseases or risk factors for cancer and the like, this would be invaluable as it'd prevent a lifetime of pharma dependency and/or prevent early deaths.

As for fertility, certain genetic deficiencies make it highly unlikely that a zygote will ever make it to term. This might allow us to correct those deficiencies and increase a couple's chance to conceive and make it to term.

Plus, we could make people glow, which would make midnight gymnastics more fun, especially if parts glowed the hotter they got...

Or insert a gene for bigger boobies into all women. Probably more important than curing diseases, actually.