they're usually used to buy collectibles, the pixels you see sometimes under people's avatar. you can also use them to buy acnl town signatures, and art of your mayor/oc.
Also, if you trade a lot (villagers, stuff, hybrids, etc.), they're convenient form of currency. For example, instead of dropping like 20 mil of in-game bells to buy a villager, it's easier to just transfer TBT.
Also, a lot of art whether your mayor or villagers, signatures, etc., a lot of those shops take TBT.
Because so many people have been asking for them, they have become quite popular, they are an easy way for you to keep all your money/save whilst you can still get new items, flowers, villagers ect.