The displacement tool does a lot that you can’t get done with the UE4 landscape tool (or any tool in UE4). Take the Antlion Caves in Episode 2, for example:
https://c2.staticflickr.com/2/1467/25601417015_cf6ba2f664_o.jpg
https://c2.staticflickr.com/2/1581/24974741413_89d29a96bf_o.jpg