TBT is faster than dropping bells. Someone may not have time for you to sit there and drop a couple ten, twenty mil. And it also prevents scammers who lowball you and don't pay full price
Lurking means your sitting on a thread refreshing it to make sure you are the possible first person to claim a villager when they go in boxes. Basically you're keeping an eye on them.
I never thought I would care about tbt bells, but I found they sure come in handy when buying or reserving a villager. They're also great for tipping.And I have sold them for in game bells a few times. They're very useful.
Lurking just means that this is a thread that I might post on later or this is a thread that I might buy something from. It's basically like a subscription to a magazine or even like YouTube, it'll indicate that you might be back later looking for something or someone like a villager.
TBT honestly doesn't really do much besides let you buy collectibles from the shops since some people are obsessed with getting those. But sometimes it's easier to just buy a villager that might cost millions but only a few hundreds of TBT because of the exchange rates. It's just much better to give TBT in one shot than to go back and forth doing the re-tail or dropping bells method.
Some people also use it for art like signatures and avatars, it's just like any other currency system really. It depends on what you will use it for. Sometimes if you have enough you can even buy real games with the TBT. Plus a lot of people on here probably have way more than they need in in-game bells and those aren't useful except for in the game either so it's the kinda same deal.