The more languages you know, the more ways you can understand people calling you stupid :D

The reason to learn any language varies from person to person, there is no solid answer to learn any specific language beside to understand it.

But not with the same people then! The language will open you a new door to communicate with various tongues and nationalities, hence it will widen your horizons and extend the range of subjects you can talk about. So it won't be with the same people and it won't stay the same subjects.

Learning a language is all about communication and it doesn't matter what subjects you talk about.

First of all, you will be able to talk about those subjects anywhere. "Taxi language" is the same from one language to the next (turn left, go straight, etc.), but if you don't speak it in the language of the country you are visiting, then you'll encounter more difficulty. Secondly, being able to ask the same questions and have the "same" conversations with people in different languages (and therefore likely countries and cultures), you will start to learn and appreciate more the similarities and differences between cultures. Subtle cultural differences are evident in the small talk - the general subjects might be the same, but the learning is in discovering the differences.