They are souvenirs, sort of a custom of "share the love" like giving gifts at Christmas time. You don't need to gift back in return, but some tiny snacks would be always appreciated. If you really want to you can bring something back from your trip to share with your office mates.
Other times when sharing little snacks or gifts are appropriate:
1) Wedding (think edible wedding favors for those who can't come)
2) Funeral (if you attend there's a meal at the end, sort of a "time to stop grieving" meal)
3) Specific national holiday like Harvest Moon, New Year, etc etc (sort of like a mini potluck)
4) Children's wedding (see 1, except it is more formal because you are now "elder" and you need to act appropriately)
5) Children's month old or 100 day old birthday (celerbration that the kid made it pass the critical point, he or she will live)
For 1,2,4,5, in many culture you should give them a small gift in cash (i.e. $20-50 for acquaintance, $100-300 if very good friend or relatives) or whatever appropriate in physical store with receipts (i.e. think Wedding gifts). In the Western world they know you are not living the custom so they won't expect anything, but small gifts are always nice.
Details depend on the local custom, as you know Asia is kind of big.