“Why are some fillings replaced?”

Hard to say without seeing the filling in need of replacement, but if the adhesive that holds the filling in place is failing it can be leaking and this leads to discoloration or staining around the filling. If this is the case, then it would be proper treatment to replace the restoration (filling). This can happen for a number of reasons, but I would ask your dentist why they feel this has happened.

Fillings are replaced when there is something wrong with the filling. You may have been bitting on something hard. Ice chewing can fracture a filling. Sometimes a cookie or cake can have a piece of a nut shell. Cherry pits. When a filling is cracked, food particles can get between the cack and the tooth, leak down to the bottom of the filling. You can't CLEAN there and
the tooth can decay. The filling materials today are great. Using your teeth like tools is risky. As we get older, our teeth can become brittle. If you are biting on something hard and you get that feeling, maybe you should stop -- listen to that feeling.

I replace fillings when they fail. To me a failure is a fractured restoration, loose restoration, or restoration with a cavity underneath it.

Fillings are replaced when the margin of the filling has lost its integrity. The margin of a filling is the boundary between the real tooth and the filling material. When the margin of the filling is weakened, bacteria can get into the tooth and affect the nerve. Staining at the margin of the filling is an indication that the margin is not longer sealed from bacteria. Staining around a filling can lead to a cavity and possibly eventually the need for a root canal.