Some of those pictures could be the only copies. Losing them could be devastating, depending on the person.

Of course, I understand their value to the homeowner, but I fail to see what value they might have to a burglar.

Cutting paintings from frames allow them to be rolled up and be easily transported (much less obtrusive or easy to spot compared to a picture in a frame). They can then be fenced like other merchandise.

A random act of cold and calculated malice, or the burglars know the homeowner and wanted to destroy as much property as possible.

To look for things hidden between pictures and backings.