@Jabra:
Bounty is an indicator of strength - unless told otherwise. It's not a powerlevel, but Oda definitely uses it as a tool to show in which ballpark a pirate is playing. Especially since the timeskip where new and powerful people show up every few chapters, with little or no time to establish their threat through actions first.
Robin & Blackbeard are exceptions proving the rule, the Shichibukai have their bounties frozen (super convenient for Oda to avoid early bounty creep) and the Straw Hats are also excluded - because we, the readers, know them so well that Oda doesn't have to rely on a number for us to get an impression.
Now Perospero said his bounty was 700m. We as readers immediately gained the impression that this guy is serious business = the tool is working. It doesn't mean that he's 150m or 300m points weaker than Smoothie or Katakuri, but that he's "up there".
The bounty system is clever and so much more convenient than a typical powercounter, despite being the same tool. Don't deny it, give it credit.
Basically this. It's a story-telling device more than anything else. Think about Jack.
Oda didn't have enough time to setup how seriously we need to take this character before the story demanded him to take one L after another. Besides wrecking the Minks, which he did mostly through the use of a chemical weapon, we've really only seen Jack get his ass handed to him.
Which makes his ridiculous bounty so important. The $1B is shorthand for "this is not someone to mess with" and it helps give weight to the fact that Zunisha crushed him.
If you really want to get technical with it, which I wouldn't recommend with really anything that happens in One Piece, think about it from the Marines' perspective. At a certain point, the amount of money someone is worth really stops mattering. If you're strong enough to take down someone worth $1B, you're probably not doing it for the money.
There are less stressful and dangerous ways to make money in OP. How many bounty hunters have we seen aim for guys with bounties in the hundred thousand range?
It's about status. The Marines are communicating to everyone that this person inspires this level of fear.
Oda uses bounties much in the same way.