The silicone molds have been used by pastry and kitchen professionals for about thirty years. They render immense services both for moldings (chocolates, jellies, ice creams, creams for desserts, etc.) as for unmoulding cakes without hindrance, madeleines, financiers and other cupcakes such as muffins or mini cakes.
Very practical too: the silicone sheets to place on baking trays to avoid the use of baking paper.
15 years ago, we saw them appear in the field of the general public, first in a discreet way then on a larger scale. They are part of our daily life today.
The promise: allow easy demolding, without greasing the mold.
However, I have seen pros grease their silicone molds.
So: should you butter your silicone mold or not?
Apart from the brands which recommend doing this during the first use, it is not useful to butter them or grease them with oil. This is what makes this material so strong.
On the other hand, some pastry chefs recommend doing this if you use it intensively (as is the case in pastry laboratories) in order to extend their lifespan. If you take out your silicone molds only for your Sunday cakes, you can happily skip this step.