In theory it's to save server hits, buying a reasonable amount after a single mall search instead of buying them one by one, one mall search at a time.
EDIT: just to be clear, Mafia bulk purchases only if you can interact and if you have more than 5k meat on hand.
These are the items that are always purchased in bulk:
- soft green echo eyedrop antidote
- tiny house
- scroll of drastic healing
- anti-anti-antidote
Furthermore, any multi-usable item and HP/MP restorer purchased while you are auto-adventuring will also be bought in bulk.
