Why would a decoder use StructTag.Lookup("config") instead of Get("config")?

Why would a decoder use StructTag.Lookup("config") instead of Get("config")?

1. Lookup distinguishes a missing key from a present key with an empty value.
2. Lookup validates the tag according to encoding/json rules.
3. Lookup returns the field index path together with the tag.
4. Get cannot read tags on exported fields.