These have many benefits over silver fillings. They bond to the tooth and so provide some support to the remaining tooth. This can prevent leakage as they seal the edge of the tooth and prevent bacteria infiltrating the tooth.
They do not corrode in your mouth or release mercury vapour.
They can be kept very small as they do not need to be extended to provide mechanical ways to keep them in.
They can be easily repaired.
They look good!
If fillings are going to be very large it may be necessary to have a porcelain inlay (This is a filling that is made in a dental laboratory and then cemented in place). It may be too difficult to construct a quality restoration freehand in the mouth if it is large.
Are white fillings weaker than silver fillings?
My experience of white fillings is that they are extremely strong and rarely break. As they bond to the teeth, I have found that the incidence of teeth breaking is very low. With silver fillings the tooth around the filling often breaks requiring further, more extensive treatment. If the tooth breaks around a white filling, it can often be repaired by bonding more of the resin to it.
I believe that resin is now the filling material of choice and that amalgam has had its day!