Electronic methods of translation are becoming increasingly sophisticated, and most people prefer to use them or an old-fashioned phrasebook and dictionary when performing such tasks as shopping or ordering at a restaurant in a place where they are not fluent in the local language. In the majority of situations, an interpreter would be intrusive, as well as an expensive luxury, preventing one from learning the language oneself and interacting with local people.
Two examples of situations when you would need to seek assistance from an interpreter are if you are arrested for a crime and if you have a serious medical problem requiring treatment. In both cases, the substantial issue is the same. There is technical language to be negotiated, and detailed explanations will be required.
Both situations are stressful, and you will need to deal with the legal or medical issue without worrying too much about the linguistic dimension of the situation. The interpreter will be able to explain to you exactly what is happening and convey your views and concerns to lawyers, doctors, and other professionals. Other situations in which you might need an interpreter's assistance are ones that have similar features: situations where a great deal is at stake and complex terminology is likely to be used.