Kubernetes sidecar containers are used to assist primary application containers by handling tasks such as logging, configuration management, and network security. Sidecars run alongside the main container in the same Pod and can be scaled independently of the main container. They offer benefits like durability, scalability, modularity, independence, and reusability but also present challenges such as higher resource consumption and cost, debugging complexities, potential latency, and the need to configure them with care. To implement sidecar containers effectively, it's essential to follow best practices like assigning a single responsibility principle, creating small and modular apps, setting resource limits and monitoring sidecar containers, enforcing security and permissions limits, and having a reason for using sidecars. Additionally, tools like groundcover can help simplify the management of sidecars and provide better visibility into application performance.