It's eventually to do with paying of a licence -
http://www.bbc.co.uk/news/entertainment-arts-37477229
But as it's not yet mandatory or on all TV devices, for now its main function is to finally sync your viewing between all your iPlayer devices.
I suppose it's possible that once it is mandatory and for all devices, then iPlayer and BBC Store might start merging together. All depends what the BBC is allowed to do for that.