These words only add value to English because they are vague; if that were not the case, English would need only the cardinals to represent quantities. 

A vague term, by definition, has no discrete boundary between itself and its coordinate term (its semantic neighbour). The gradated boundaries of vague terms make them uniquely efficient communication tools. Consider that a vague term conveys more information than a range of values conveys: A vague term can be conceived to be a set of continual (non-discrete) values; the greater the difference between a value and the most ideal, (or prototypical) value in the set, the less ideal that value will be. For instance, the word 'bald' becomes less true the less the subject's scalp resemble's Patrick Stewart's scalp. 

Precisely representing a vague term requires many more words or much more notation than defining a range requires. So vague terms are semantically economical. 

In short, I think it's best to conceive terms such as 'several', 'couple', and 'few' to be overlapping value-ranges with no discrete boundary between any two of them.