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: Consider that a vague term conveys more information than a range of values conveys. Where a range represents a series of values, a vague term represents a set of continual (non-discrete) values; the greater the difference between the most prototypical value in the set and any other value in the set, the less prototypical that value will be. For example, a subject is concieved to be less 'bald', the less his scalp resembles 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.