I can buy a storage for 1year

As far as I know, there is no facility to pay for storage by yearly payment. Apple probably do this so that, if you find that you no longer need the storage, you can cancel the next monthly payment via online banking.

If you paid a full year's storage for, say, 2Tb storage, and then find that you would prefer to store everything onto an external HD, in June of that year, you still have 6 months of storage time paid for which may be $42, rather than $7 for the next month's unused storage.