Form customizers are SharePoint Framework components that allow you to override the form experience in a list or library level by associating the component to the used content type. Form customizer components can be used in SharePoint Online, and you build them using modern JavaScript tools and libraries.

As we know, SPFx runs in the context of logged In user. In a scenario where your application needs to perform some tasks which require more permissions then currently logged In user, you can use this approach to handle those scenarios.
For example, You can configure Azure AD app with elevated Microsoft Graph API permissions and then call Microsoft Graph API from Azure Function.