A field's data type is the rule Salesforce uses to decide what kind of value can be stored, how it is formatted, and whether the entry is valid. Choosing the type is the first step when you create a custom field, and it is one of the main ways an administrator improves data quality before users ever type a value. After records exist, converting to another type can delete stored values, so pick the type that matches the data rather than treating conversion as later cleanup.