Many times, you get requirements to add additional fields, validations, or change the layout of existing forms. Most of these requirements can be achieved by metadata extensions. Business logic on the forms can be extended by subscribing to standard events at a form, data source, or control level.
For details around user interface extensibility, go to https://docs.microsoft.com/en-us/dynamics365/operations/dev-itpro/extensibility/extensibility-home-page.