My mum does it because she's scared of changing lanes - ones of those people who won't put her indicator until everyone has past by which time she's had to stop...very silly but I do tell her "that's what indicators are for". She's maybe done 10 miles of motorway driving the whole time I've been alive
The GF does it because she's clueless and should never have been given a driving licence...too busy singing along to Westlife or something crap like that.
My brother does it too, not sure why.
Lane discipline is a really overlooked in this age of speed cameras. A little education goes a long way.