A bounty campaign is a marketing ploy to promote the token. The conditions for bounty campaigns are more complicated than those of the airdrop. The whole point of the bounty is to receive tokens for certain promotional and informational actions, in which there is no need to invest your money.
The bounty campaign allows the project to create a stable, interested group of users and reach the audience as much as possible. It is done with the help of charity events, including in social networks, advertising on blogs of micro-influencers and bloggers with a large audience.
Bounties also help in the development of projects. For example, a campaign is often aimed at rewarding those users who help in finding and fixing bugs, moderating forums, translating documentation into different languages, etc.