Valid question @feelus. CNY is one of those times of the year when literally everyone in Hong Kong (locals / non-locals) working (across industries) expects to rcv a red packet. More than the amount its the anticipation thats exciting and makes people happy.
Its also symbolic for many reasons, besides wishing luck it also is an opportunity to show gratitude not only for that visit but for all the other times you may have visited your girl / an establishment over the year. Build relationships and goodwill, it goes a long way.
If for nothing else, its a festival, spread some joy.