Exactly. It's well known that human short-term/working memory is pretty limited in its capacity. The last thing I would want a newly-qualified driver to do is to be told 'just pay attention!' when they've forgotten a signal aspect and then beat themselves up for behaving in a perfectly normal human way.Easily said.
But in practice I can think of a few examples where you pass a signal, stop at a station and the next signal is out of sight round a corner. Add in a passenger alarm going off, a TCMS fault flashing up in your face, distractions at the station or the signaller contacting you via GSMR and it's all too easy to forget what your last signal was. Particularly if it's your last trip on a 10 hour job.
Those are the situations where a glance at the AWS before setting off from the station could potentially save you from a SPAD.