These are typically ideas for new applications/tools that build on top of the core Balancer protocol or data (think of Coppers Fair Launch UI). These ideas are sourced from the community so are already something that people want and will use.
Well specified issues/feature requests which are a good candidates for external contribution. These Bounties can be good for individual contributors and can be a great way to get to know the Balancer protocol in more detail. Also Balancer is always on the look out for great developers and completing Bounties are a great way to demonstrate your skills while getting paid!