Specifically, the syncytins are important because they keep the mother's immune cells from being able to reach "through" the placenta into the developing fetus.

There are certain immune cells that are able to slip between other epithelial cells. If the mother's immune cells were to slip past the cells of placenta, they would almost certainly attack the fetus. Placental mammals solve this by having a "boundary layer" between the placenta and the mother. The cells of the boundary layer use those viral syncytins to "fuse" together, becoming one giant solid mega-cell with lots of nuclei. Because there are no longer any individual cells to slip between, the mother's immune cells are unable to get past this boundary layer (the syncytiotrophoblast) and thus the rest of the placenta and fetus are protected from the mother's immune system.