There are numbers of reasons to learn another language. English, specifically, has its own advantages. International business, aviation and media use English almost exclusively. Educational opportunities in the US, England and Canada are extensive. English is expository. It will make you a better debater, a better writer and helps you express yourself.
The disadvantages depend on the nationality of the person learning English. Chinese students see learning English as an insult to their Chinese ancestry. Arabs see the language as the sounds of an enemy. Tonal languages can't quite grasp the direct nature of the language. Some backgrounds do not see the need for individual thought or individual expression. It is a difficult language to learn with nearly every rule having an exception. Idiom and slang is such an integral part of everyday life so that figurative speech is incomprehensible to the non-native speaker.
There are over 25,000 idioms in English from hanging up a phone to having your eye on someone. In Italy a road doesn't "go" anywhere. In France you don't live "on" a street. In Germany there is no difference between "I do work" and "I am working".
English is a Germanic language with borrowed words from every country past and present. English is an advanced, developed language; no formal or informal, no tenses, except for who and whom, and only one way to say "The". In German there are six ways to say "The".
The most important benefit is getting a better understanding of your first language. You can transfer the skills learned in English and apply them to your first language. I speak French, German and English. I teach English as a second language at a university in the US. Learning another language is seeing the world through a different lens. You see the world from another viewpoint. You become a member of the world community, not only the community you happened to be born into.