When you visualize data using D3.js, you often want to show how your data changes over time or in response to user actions. This is where transitions come in. Transitions are a core feature in D3 that allow you to animate changes in your visualizations. Rather than having elements instantly jump from one state to another, transitions let you smoothly interpolate properties such as position, color, or size. This makes updates much easier for users to follow and understand.

Imagine a bar chart where the data updates: without transitions, the bars would abruptly change height, making it hard to track what happened. With transitions, the bars smoothly grow or shrink to their new values, making the change visually intuitive. Transitions help maintain context for users and guide their attention to what is changing, which is especially important in interactive dashboards or real-time data displays.

By making changes gradual, transitions not only add visual polish but also improve the user experience. They help communicate the story behind your data more effectively, ensuring that users are not confused by sudden shifts in the visualization.