Difference between SNC supplier collaboration and SRM supplier self service

I have been looking at the functionality provided by SNC 7.0 for supplier collaboration - for example collaboration on purchase orders, where SNC provided web enabled visibility of POs, and allows suppliers to enter PO confirmations and ASNs.

The functionality looks similar to that provided in SRM via the plan driven procurement scenario with supplier self service.

So, where is the differentiation between SNC supplier collaboration and SRM?