One good reason to learn foreign languages is to get access to information.
If you’re doing research into a topic there may information about it only available in another language, and if you don’t know that language you probably won’t be aware of that information exists. If you do know the language though, you will be able to find and use the information.
Knowing other lanuages will also give you the ability to discuss your research with people from other countries, and maybe even to write and speak about your work in other languages. You could also undertake research projects using a foreign language or language, which is there handy if you’re researching other languages.
The same advantages apply not just to research but to any information you might want to find.