The Ganga is the holy river of the Hindus. Why can't we simply say, Ganga is the holy river of Hindus. why should we use the definite article 3 times?
The first is because the definite article is often used with the name of rivers.
The second is because it's saying the Ganga is the only (or at least the main) holy river for a certain group of people.
And the final one is because it's talking about Hindus as a collective mass; all Hindus, not just some.
The OED says that the is normally used to refer to an individual object (or set of objects), or to mark an object as one "before mentioned or already known, or contextually particularized". It goes on to list 23 major senses, of which your example uses three:
3. b. With names of rivers, as the Amazon, the Thames
the holy river [of the Hindus]:
15. a. Where the object is defined by a following phrase with a preposition (especially of, representing an Old English genitive).
22. With a noun in the plural, chiefly the name of a nation, class, or group of people, where the = ‘those who are’; ‘the…taken as a whole’.
As others have said, the has many different uses, and can be complex. But your specific question has a specific answer; the Hindus here means all of them, considered as a group, while Hindus means 'in general'. The same applies to other demonyms: 'Germans often go to Spain on holiday' (not the Germans, which would imply they all migrate en masse). but 'The Germans have provided a large loan to the Greeks', because the German government transferred funds from German taxpayers; it wasn't a voluntary whipround.
My Hindu theology isn't strong, but if the Ganges is so central to Hinduism that you can't be a Hindu unless you regard it as holy, then the Hindus is right; if it's only a tradition, then Hindus is equally possible.
I think it's quite true that "a" and "the" often add no meaning to a sentence: they're just required by convention. But the thing about conventions is that they are conventional. If you don't use them, people are confused, or think less of you in one way or another.
It's like shaking hands or saying "hello" when you meet someone. Does this accomplish any truly useful purpose? Not really. But if you don't do it, people think you are being rude or at least a bit odd.
Language is a pile of conventions. Why are the letters prounounced the way they are? Why do we have a different verb form when the number is one but not when it is, say, six? Why do we call that canine creature a "dog" and not a "fuzzbar"? Etc etc.