Hello,
Thanks for your question. With regards to studying the English language as a subject, it depends on where you are studying from!
If you come from a country where English is not the main language, it is very useful to study English because English is so widely spoken around the world. Most places that you go in the Western world, people speak some level of English, and it is generally the one mutual language between people. A lot of things are also written in English in the Western world, e.g. Sign posts and directions, and when there are so many thousands of different languages, it is necessary to have one communal language that everybody can understand. Therefore, it is a good idea to study it!
If you are English, or live in England (or a country where English is the first language), it is still a good idea to study English to understand the etymology of the language. When we learn the reason for language, it's inception and it's formula, we find it easier to practice and explore. The more you study English, the better you will become at it, and the better you are at it, the further you are likely to go in certain feilds!