Where guardrail protects a run, a bollard protects a single point, a doorway, a building corner, a piece of equipment, a fire hydrant. It's the simpler, faster answer whenever the hazard is a specific spot rather than a whole boundary line, and it comes in far more configurations than a guardrail run does: fixed steel pipe, removable for occasional access, spring-loaded to bounce back after a light hit, or folding down flat when the space needs to be clear.
Material and mounting follow the exposure. Steel pipe bollards take repeated heavy impact; plastic and flexible options mark a boundary without the cost or weight where impact risk is lower. Decorative and architectural finishes are available where a bollard needs to look intentional, not just industrial.