It's not necessary, but more of a convenience thing. For example, if you are from West Bengal and you only speak two languages, Bangla and English. Suppose you went for vacation in some other state and are need some urgent help. They obviously won't know Bangla. So you ask for help in English. And what if the person doesn't understand English either? You have no other choice. You'll have to keep looking for someone else who knows English. Anywhere you go in India, you can speak in Hindi. Even if they can't speak Hindi, they can definitely comprehend what you are asking for. And suppose if they understand your question in English, but they give the reply in Hindi because they can't speak English, then without knowing Hindi you won't understand what they are saying either.
In my case, I had to learn 3 languages other than my Mother tongue (Bengali). Assamese (I live in Assam), Hindi and English. So I know 4 languages in total. Now if I move for work to another state, say Hyderabad for example, then I'll have to learn Telugu as well. Which will take a year or two (or even more to perfect). Which will make it 5 languages in total. But till my leaning is over, I can make do with Hindi. Having to learn only 3 is still far better.
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