My guess would be for people who are undecided to help make up their mind. So people don't give up soon after training if they decide its not for them, or the RN catch people who would like it but weren't sure.
HMS Raleigh feedback suggests that those attending Phase One training having completed an RNAC are much better prepared physically & mentally with regard fitness, swimming ability, the ability to iron their kit, etc. The upshot being that less RNAC attendees leave in Phase One training. In addition candidates who aren't sure about joining get a good insight and those in need further evaluation for suitability for service get a fair assessment which is fed back to the AFCO if requested.
The fact is they aren't compulsory, they cost money & non-attendance is not viewed negatively, but the advantages supposedly outweigh the disadvantages from both sides.
I can appreciate that they do cost a fair bit of money and effort to organise - not to mention the hassle of having to baby sit civilians - and having had to look after work experience people I know the hassle and that was only in an office environment!
The reason I ask, is that the invite my son was sent said it was more for his benefit than the RN's but I wondered whether it could also (informally) be used as a guage to whether he is suitable - after all the recruitment process can't be cheap either!