What is the advantage over a conventional brush?

The material stays in a closed container and is therefore shielded from air and light. The amount applied depends on the set pressure rather than on the hand of the operator.

What matters about the bristle material?

The bristles sit permanently in the medium and in the cleaning agent. The material has to suit both, otherwise they swell or harden and the application becomes uneven.

When is a brush better than a needle?

Whenever a thin film has to cover an area: primer before bonding, flux before soldering, release agent before forming. A needle would need many parallel passes, and the transitions between them stay visible.