Learning a second language is great for travelling and getting a better paying job. It can also make your brain healthier and prevent the onset of Alzheimer's disease.
This TED-Ed video clip explains the different type of multilingual speakers and how speaking multiple languages helps your brain stay healthy. Generally, the earlier a child can start learning a language, the better, but even adults who learn a second language can benefit from it.
Your brain can show more activity and a higher density of grey matter that contains your brains neurons and synapses. It's also possible that knowing and speaking a second language on a regular basis can help delay the onset of Alzheimer's and dementia.
Essentially, speaking a second language gives your brain a workout that can help make it healthier in the long run. Considering that learning a second language might also make you more money, there's no reason not to consider it.