Why should Catholics do things that, strictly speaking, are not demanded by the Church or the Ten Commandments?

For instance, avoiding worldly influences such as advertising, TV, movies, video games, music, etc.?

Because possessing the Faith (a gift from God that he must give each man) is like owning a castle. Owning a castle is better than sleeping in the gutter, and it's great when your castle isn't under attack.

BUT...when you KNOW FOR A FACT that enemy armies are in your area, and that your castle is on their hit-list, you do well to build a series of moats/walls around your castle.

Not watching TV is like erecting a big wall. Daily spiritual reading  is like digging a moat. Daily examination of conscience is like adding piranhas to the moat. Daily recitation of the Rosary is like adding man-eating sharks to the water. Etc.

Matthew