Catch errors and suggest a correction
I’ve seen examples where you could type an address with a street number (only a street name), which is invalid. So depending on the quality of the address finder, you may still want to have an address validator to catch invalid addresses that the Address Finder allows.
If the address validation fails, suggest the corrected address if possible (and highlight the suggested change). Otherwise, give them the option to use or edit either the provided address or the suggested one.
Note that if you’re using an Address Finder field instead of the standard address fields, you should not need an address validator. However, the address finder isn’t necessarily perfect.
According to Addressy, 65% of smartphone users prefer type-ahead address verification (a synonym for an Address Finder) above all other forms.