Passing it gives a discount on your insurance & depending on where you are it can be done for free. I put in for the Irish one before moving to the UK, plan on doing it over here it's a mile down the road from me & £10.
There is another decent training set up which is aimed at training bikers (& drivers) to pass the RoSPA advanced test. Most private instructors can charge quite a bit, this is free for members (membership price varies among associations, highest I've seen is £55, that includes a copy of roadcraft & a couple of other books & subsequent years are £20)
http://www.roadar.org.uk/