I once had a source come to me with information about how families were being shipped in to my town from another city because the housing crisis affecting the UK meant local councils could not house residents locally.
In another case, I was given salary details for council employees who were being paid in a similar manner to this: http://www.telegraph.co.uk/news/2016/08/02/nhs-chief-on-reco....
In one other case a council employee told me how morale was low in the council's social services department due to pressure from senior officials and the workload. The source said there were concerns over how this was affecting the welfare of the children they were meant to look after.
In all three cases the stories checked out and in the published story I wrote that the initial information came from an unnamed source. In the housing case I was able to speak to some of the families and they went on record. In the payroll case I published the salaries and the council did not say my information was false. In the social services case they gave us a statement.
The problem with using anonymous sources comes when journalists use them as an excuse for publishing unverified information. If my sources had gone on record they would have most likely have lost their jobs meaning future whistle-blowers/sources would not have spoken to me or would have not spoken to other journalists in the future. If that happens how can institutions and individuals be held to account?