I think its totally daft to let the kids see them beforehand. When our daughter was doing the dreaded tests last year I recall her saying some other children in her class had been set the familiarisation papers by their tutor. Surely the whole point is to see how they perform under proper test conditions with a paper they haven't seen before.
Yes, I agree it is daft. The motivation tends to be two-fold. Firstly, the kids can brag like mad about having scored 80/80 in the practice papers and, secondly, the tutors live in the naive belief that at an appeal hearing the panel will be impressed by those 3 scores of 80/80, so little Johnny's substantially lower scores on the real papers must simply have been an "unfortunate blip" (probably involving the demise of a hamster