Do you want to make some custom changes to your UDM which Unifi hasn't released yet?
Would you like to make some changes that on EdgeOS would be achieved with config.something.yml?
If you just make changes to the interfaces, Unifi will overwrite what you've done periodically/next time you change something.
Luckily we can hook into Unifi's state file and perform the updates immediately afterwards.
For example, [configuring two IP addresses on your WAN interface, so that you can get to your modem's configuration](https://community.ui.com/questions/Access-modem-connected-to-USG/db5986b8-26cb-4d66-a332-2ace81ac8c4f#answer/7da28d8d-25c8-4ca3-b455-c6eba836f034).