The once-only principle would save a lot of time to citizens, businesses and public administrations. It has many advantages.
Until now, it has usually been the case that documents have to be submitted in every authority, i.e. it is always the same information that is passed on to the authorities. This procedure is paper-based and involves a great deal of effort.
The once-only principle is based on the premise that standard administrative information only needs to be communicated once. The different administrations then exchange the information with each other. For citizens and businesses, this means less administrative contact and faster administrative services through data exchange. Once-only also increases transparency , for example, if it is possible to trace which authorities have requested which data in user accounts.
For public administrations, this principle brings more efficiency, as media disruption can be avoided and costs can be saved. In addition, the data is of high quality and forgery-proof, for example if there is a blockchain behind it. At the same time, there is networking a knowledge exchange between the administrations that can have a positive influence. Currently, there are also projects to promote the once-only principle across borders at EU level. 
Once Only, however, is accompanied by legal, technical and organisational challenges that have to be solved.
 The Once-Only Principle Project (2017): http://www.toop.eu/, retrieved June 20, 2018.