Kripke uses it as a concrete demonstration of (his interpretation of) Wittgenstein's rule-following considerations, framing them as a skeptical paradox. The problem is, if you have never added together numbers higher than 50, all your additions are compatible both with taking "add" to mean "mathematical plus" and with taking "add" to mean "quus". It may be that everyone asking you to add things together has really been meaning you to quus them all along, and you've been plussing them. It's only when we deal with numbers over 50 that we start to see that plus and quus are different, and the question arises of which rule we are meant to follow when we are told to "add", and we discover whether we have been following the same rule as everyone else. But more than that, in what way is it true that we have been quusing when they have been plussing, because in what have they following the plus rule rather than the quus rule? Not because of what they did, since what they did was compatible with both rules. And not because of what they thought, because it's possible that they never even considered what they would do when dealing with numbers over 50 (have you ever consciously considered how you would answer 547+789? Maybe you would answer '5'. You probably wouldn't, but that fact doesn't come from what you have consciously thought before about what you would do in this situation). And of course you can't try to explain by using other rules (like 'x+y means give the yth successor of x'), because those rules are themselves subject to the same ambiguities, and indeed in the case of mathematical rules are just restatements of the problem in other words ('the successor of x' is no less ambiguous than 'x+1').
So how are we ever able to learn, and use correctly and in the same way as everybody else, rules that cover an infinite number of different circumstances, when we can only learn from a finite number of circumstances, and when an infinite number of eventually-conflicting rules are compatible both with our finite experiences and with any attempt to describe the rule in language?
"Quus" isn't a very commonly used word in philosophy (nowhere near as common as 'grue', I'd wager), but people do still talk about it.
But the river tripped on her by and by, lapping
as though her heart was brook: Why, why, why! Weh, O weh
I'se so silly to be flowing but I no canna stay!