What is the use of Invitee Questions in Event Type customization?

Using Invitee Questions in event type Customization, you can get all the info you need from invitees before the appointment. You can customize your event type by adding any number of invitee questions. 

Steps to add invitee questions in an event type:

1) Go to Event Type customization page and then go to "Invitee preferences" section on that page.

2) Click the highlighted button "+ NEW QUESTION" to add invitee question

You can choose from a variety of field types ("Answer type" field) including text, checkbox, radio button, and drop down (list box). 

You can also mark which invitee question is optional and which one is mandatory using the red color highlighted checkbox in the below image.  You can add a new answer option using the blue color highlighted button. You can delete an answer option by clicking the green color highlighted icon in the below image.

You can reorder questions easily using drag and drop approach - follow the approach mentioned in the red color highlighted text in the below image to do that. To delete a question, click on the green color highlighted icon in the below image. To modify a question, you have to delete a question and add it again.

Answers specified by invitees to the invitee questions are shown in the calendar invite, appointment notification emails and appointment details page.