When translating a page, you have to pick a language to translate to but can't select the language to translate from.
- A translator if fluent in two or more languages, but not English.
- The original is in Korean and a good English translation is available. Most translators would probably rather translate from English as Korean isn't very popular as a second language.
- Translation into a related language. For example, pt is already available and someone wants to add pt-br. (Brazilian Portuguese) They would rather translate from pt instead of en. If someone wants to add fy (Frysk), they would probably rather translate from Dutch than English.
Update: Sorry, I goofed up. I meant the Translate extension, not ContentTranslation.
(Despite it being so easy. The difference is easy to remember as ContentTranslation is used for the Translation of Content while the Translate extension is used to translate... non-content?? I need a new mnemonic)