A destitute man, if he is not supported by the parish, can only get relief at the casual wards, and as each casual ward will only admit him for one night, he is automatically kept moving. He is a vagrant because, in the state of the law, it is that or starve. But people have been brought up to believe in the tramp-monster, and so they prefer to think that there must be some more or less villainous motive for tramping.
