Hrqls: the only real reason i can think of would be to study ancient texts in their original language
Or to better understand how a certain language evolved.

For instance, if you study latin, you may have an easier task finding similarities between french, spanish, italian and portuguese and understand them easier, since you'll know a strong common root between them.
(It doesn't mean you'll know how to speak them well. Linguists in so many languages are rare - but you'll understand them fair enough)