I believe they were rather to protect themselves. My father fought in the trenches for four years with the Argyll and Sutherland Highlanders, and he told me his khaki "apron" was needed at times for very necessary camouflage, especially when they crawled out in to the mud of no-man's land and had to assess enemy trenches, as he did.
I have a photograph of him wearing his "apron".
Lynne Arnott
Redford Loan
Edinburgh