Business English is the English used in business contexts, such as foreign trade, commerce, c, marketing, human resources, and many office settings. It allows one to be more professional and effective in their business life.
There’s a lot more to learning a second language than mere vocabulary and grammar. Languages are used differently in different contexts. For example, you probably speak to your boss differently than to your grand-parents. We all write differently for different contexts as well. A text message is certainly not going to be the same as a formal business email.