Lots of people speak English so one of the reasons to learn it is to interact with more people. You could also watch films without subtitles or read books that aren't translated to other languages.. but the most used and most important reason is work. For most jobs you need English even if you live in a country where English isn't the main language. You need it to interact with customers and colleagues.
If you want to work in another country you obviously have to learn the language of the country too, or if your just visiting it is helpful so you can understand what the people surrounding you are saying.