The RNR isn't intended to provide an insight or as work experience however it does actually fulfill that particular function and provides a part time income to augment your finances during your time in education.
The RNR, for their part, are usually quite happy to have people join them for a couple of years because a fair few actually end-up finding a well-paid civilian job and then remain in the RNR.
It is theoretically possible to transfer seamlessly into the regular RN from the RNR with minimal AFCO involvement as all (or most) selection elements remain valid.
Potentially, a bit, if you come at it with the right attitude. You won't advance faster or gain seniority based on "time done in the RNR", but you may find it's less of a culture shock when you go from "civilian" to "servicebeing" via "reservist".
Firstly, the RNR isn't set up as a pre-entry course for the Regulars; however, unless your local unit is at full capacity, they'll generally be willing to take you on for a while until you join full-time, since (a) you'll hopefully go on to say nice things about the RNR, who most Regulars rarely run into, (b) you may decide against going full-time and remain a useful member of the RNR, (c) better to have someone keen if transient than an empty place in the unit. For an example, we had one good guy in my last unit, who had passed AIB but hadn't got the score he'd hoped for; for about a year he made himself useful, was good company, then he retook his AIB, scored well, was selected and is now doing well in the regular RN.
Secondly, the RNR is a different beast in several ways from the regulars, so while you'll gain some useful skill and knowledge, you'll still have to check and adjust when you go Regular. Don't be the "but we didn't do it like this in the RNR!" person, nobody likes them.