Translation is customization, kind of 
But really widely used for translating anyway.
Finnish translations are quite good, but not suitable for me everytime. Then I should not try to change translation itself, but do some customization. Plus it is quite much faster way.
Actually I have to do quite much translating job after every plugin/theme/component and I have still text in english, even on/in/at (I really really hate prepositions) as some warnings from composer. Sure, I can/should do it via official translation, but again — I want it be translated now, not someday. And then option to show not translated strings helps.
The main question is not all possible ways to do translations, but is such option too hard/expensive to code. Other options are good to know, though.