It's to show how casual and almost careless Guenevere is.
The process of “tearing” the fresco off the original wall is called Strappo. Fresco means “wet”. Paint is applied to wet plaster and becomes part of the plaster. This allows the fresco to look virtually the same for over a thousand years, so long as it is not exposed to water or sunlight.
This is a message, usually about life or society, that an author wishes to convey through a literary work. It is called the central idea, main point, or systematic argument.