first you have the problem of some calculations beeing too large for conventional computational capacity these equations are classified as BQP or bounded-error quantum polynomial time and the best way to formulate the question is what is the use of quantum computing and that's where a qubit pops in literally

Some calculations are too large for conventional (classical) computational capacity. These problems are classified as BQP, or bounded-error quantum polynomial time. The best way to formulate the question is "what is the use of quantum computing?", and that's where a qubit becomes useful.