IT has been great at coming up with new ways to protect your entire network from any external source gaining access to your data. This is definitely a great and necessary layer of defense to have in place. This has been the go to method in securing networks since we have had them. The issue is that not all attacks come from outside of the perimeter.
Most perimeter security devices are just that perimeter security devices. They do not protect the data that is stored on servers or workstations from any threat that has already made it into the network. Threats can come from anyone at anytime, internal or external. Having your data encrypted does add another layer of security, however that data is only encrypted when it is being accessed from an untrusted source. Your employees would probably have access to this data in an unencrypted format. Onions have many layers of security when protecting there seed, which is what you need to do when protecting your data.
This is why you need to look at a defense-in-depth approach when securing your data. A real world example is Edward Snowden, he took confidential data from his employer without their consent. This does happen at every level of business and more so when it was not expected in the first place.
There are many additional layers of security you can implement including, physical layer, network layer or the perimeter, application layer, platform layer, and the user layer. If you apply security to all of these layers you will have a defense-in-depth approach to securing your information. The image below demonstrates the different levels of security.