Adding custom controls to a Leaflet map in a React application gives you the flexibility to tailor the user experience. Instead of relying solely on Leaflet's built-in controls, you can create your own buttons, sliders, or other interactive elements using React components. These custom controls might allow users to toggle layers, zoom to specific locations, or filter displayed data directly from the map interface. In React, you typically render these controls outside the map container and use React state to manage their behavior. This approach keeps your UI consistent and leverages React's reactivity, ensuring that custom controls update in response to state changes or user actions.

Synchronizing map interactions with other parts of your React UI is essential for a cohesive user experience. When a user clicks on a marker or selects a region on the map, you might want to update a list or filter panel elsewhere in your app to reflect that selection. Similarly, adjusting a filter in a sidebar could update the visible markers or layers on the map in real time.

This synchronization is achieved by lifting relevant state up to a common ancestor component, allowing both the map and other UI elements to respond to changes. By sharing state and passing down event handlers as props, you ensure that the map remains in sync with lists, filters, or any other interface components, creating a seamless and interactive mapping application.