Last tested: Jun 1, 2020
type: of a dimension impacts two things:
1. The filter dropdown options list
2. How the values are rendered on the front end
For filter options, you'll notice the list of available options for a type: date field differ than those of a type: string -- Looker determines which list to use based on the type: parameter defined for the field.
The type: parameter does not do any casting. This is a common point of confusion for customers.
If you don't specify a type: parameter, the type will default to string