The DHT of a Holochain app makes sure that there are always enough nodes on the network to hold a given piece of data.
When people running Holochain apps turn off their device, they leave the network. What happens to their data and the data of other people that they were storing? There are always enough nodes that hold a given piece of data in the network to prevent data loss when nodes leave. The DHT and Holochain gossip protocol are designed this way. Also, the redundancy factor of data on a given DHT is configurable, so it can be fine-tuned for any purpose. For example, a chat app for a small team might set a redundancy factor of 100% in order to prevent long loading times, while an app with thousands of users might have a very small redundancy factor.