• When you need extra validation of the input data.
  • When you need to visualize certain fields differently. For example, if you need to specify a location, you could visualize it with the Bing control.
  • When you there is a need for cascading dropdowns (list options in a second dropdown must be limited based on the selection made in a first dropdown.
  • When there is a need to hide or show certain sections on the form, based on a selected option

It is very simple to start the creation of a custom form: on the list view, select the PowerApps button and choose Customize forms:

This will open the PowerApps canvas where you can start customizing your form.

There is also a change in the List Settings to give you the possibility to switch between the default forms, InfoPath forms and PowerApps forms: go to List Settings > Forms settings:

To start with this feature, you can read the following:

https://powerapps.microsoft.com/en-us/blog/q3-2017-update/

https://docs.microsoft.com/en-us/powerapps/customize-list-form

This feature is only available in SharePoint Online, and only for lists; not (yet) for document libraries.