Many were used in computer control applications, and many educational establishments used them as general purpose computers.
The most prominent application of them relates to the origins of the internet. The DDP-516 was used as the basis of "Interface Message Processors" or IMPs that were used to connect the very first networked computers to the ARPANET.
The story of the development of the ARPANET is described in a book: Where Wizards Stay Up Late, Katie Hafner and Mathew Lyon, ISBN 0-684-83267-4
- The Senster