Userforms are custom dialogs used to collect input data from the user at the start of a macro. The UserForm Builder included in Code VBA automatically makes UserForms.
The generated user forms have with input validation code added to ensure the all required input is of the correct type. This will make the macro more reliable.
Having the controls and validation code generated saves a lot of time you would have to spend to create the userform manually.
The UserForm can be changed both to make the design visually more attractive by moving and resizing the controls.
In addition you can extend the userform vba code to fulfill extra requirements.