For most types of fields, you can assign a default value. A default value is one that is assumed when no other data has been entered. For example, if you defined the custom field "Preferred Swag" as a list of specific choices, and you know that most of your volunteers are going to want t-shirts rather than baseball caps or sweatpants, you could set "T-shirt" as the default value for that field. Anybody who is entered will then start out with the value "T-shirt", and you only need to edit those people who want something else.