As you know, there are different dialects of English around the world like British English, American English, Australian and so on. In most cases it does not really matter which English dialect you learn as long as you try to stick with it and don’t mix up different Englishes. When you learn English as a second language in school, in the vast majority of cases, you’re either taught British or American English.
However, the situation changes when it comes to studying in London or working in New York City. Dialects, accents, a slightly different vocabulary and grammar can play an important role as proper spelling might have an effect on your grades at university or your coworkers will simply understand you better when you talk the way they are used to, may it be in a British or American accent or the use of certain words and vocabulary .