Why is unofficial required then?
A translation agency doesn’t provide just translation for immigration but much more: translation for personal reasons and translation for business, which includes fields like medical, technical, etc. Such translations are not supposed to be submitted for official purposes and do not need a certificate of accuracy.
Another type of unofficial translation that is highly used these days is website translation, used by companies in order to expand to certain markets. Software localization is also unofficial, yet it is needed in order to translate a software suite or an app into another language.
There are many fields where the translation, although highly accurate, doesn’t need a certificate of authenticity. When it comes to immigration or getting a job in another country, certified translation becomes necessary.