Two scenarios:
Barman please pour me 5 glasses of wine and please use 5 vouchers.
versus
Barman please give me the bottle of wine and please use 5 vouchers.
Its easier, it saves me going to the bar 5 times, it is less work for them as it is only one transaction, which leaves them more time to serve others, and it is less washing up as only one glass is used not 5!!!