While Charlie was originally selected as preferable to a real living animal or human, in much the same way and for much the same reasons that armies will use hanging sacks of straw for bayonet practice, the downside of this is that Charlie has absorbed so much directed witch magic over the years - including having had a previous head exploded by Granny Weatherwax - that he has now developed a kind of sentience. In Lancre, it is thought of as slightly worrying, if not bad luck, to have Charlie suddenly appear in your garden or at your front door. Yet nobody has ever seen the scarecrow actually move.
He doesn't worry Nanny Ogg (well, what does?) She once found him in her bedroom, still on his stake. She says that if you're friendly and polite to him, you may get a bumper crop of pumpkins next harvest, even if you didn't plant any.