It's so that you look respectable in arial photographs and satellite imagery etc..
I had this conversation with my Nan..
Nan - what if Mrs Brown at no 47 is a member of the secret service..?
Me - what makes you think that Nan..?
Nan - why are you answering a question with a question..?
Me - sorry.. it just seemed a wierd thing to be concerned about..
Nan - well stop being so bleeding shifty.. I'm your Nan
I should add at this point she gave me a sharp clip around the ear and her eye shape changed from narrow and suspicious to steely and defiant..
Nan - well.. if she is a spy.. I won't let her get the better of me..
Me - what are you talking about Nan..!?
Sharp clip around the ear
Nan - I've arranged to have our roof cleaned so that she can't look down her nose at us when she's analysing data from satellite photography while she's at her fancy job with Mi6
Me - Aw that's a brilliant idea Nan.. you're so clever.. can I have five quid for a mars bar now please..?
(mutter under breath - mad old witch)
you can tell which people starch their underwear in my street by the cleanliness of the roofs..