As we know Bill Weasley works as Curse-Breaker for Gringotts in Egypt.
Gringotts is a bank. Which part of their business would require a Curse-Breaker?
Speculation, but I'm not sure there's any canon answer.
We know that whatever "curse breaker" is an active, in the field job. We know this from Order of the Phoenix.
We can also infer it's likely related to the "cursed" Egyptian tombs, not only from the above quote but from Prisoner of Azkaban.
So it seems the tombs really were cursed by ancient wizards.
Final piece of lore that helps flesh this out - Goblin ownership.
Given the Goblin notion of ownership (aka the owner of an item is the crafter, everything else is a loan) that, as well as raiding ancient areas for gold, it's possible that Gringotts mounts regular excavations to "retrieve" Goblin artifacts buried with wizards or muggles.
Regardless, it seems pretty clear that Bill's most likely job is exploring ruins and "defusing" any traps (curses) left by ancient magical kind.
A Curse-breaker basically makes sure that tombs and ancient sites are safe before Gringotts takes gold from it.
Here's what the pamphlet that Gringotts gives out to fifth years says:
And here's what the wiki says:
I know there's a quote from Ron in one of the books that says exactly that. As soon as I find it, I'll put it here. (see @ibid's answer for that)
From Ron's letter to Harry, we can assume that curse breakers remove curses from old tombs to extract the gold.
Book of Spells goes further, and equates curse breakers with treasure hunters
Besides breaking the curses in old tombs, I would guess just regular banking business could make use of a curse-breaker.
The bank might often need to collect valuables which they have the legal right to, but their owners might be reluctant to give up. Imagine that you owe the bank a nice sum of money, be it from accumulated fees you didn't pay, or because of a loan. They win a case against you, and you know that collections agents will show up soon so that you have to give up money or a nice golden family heirloom if you don't have enough money. Wouldn't there be a temptation to put a curse on it? I would guess many real-life muggles would want to put a curse on such things if they had the means to do so.