Why are coloured balls returned to their spots while red balls remain pocketed in snooker?

In snooker, the rules dictate that red balls remain pocketed once they are sunk, while the coloured balls are returned to their designated spots on the table as long as any red balls remain in play.12

This distinction is fundamental to the game's scoring structure. Players must alternate between pocketing a red ball and a "colour" (non-red ball). By returning the coloured balls to the table, the game ensures that these higher-value targets remain available for the player to attempt to pocket after each successful red ball, maintaining the tactical flow of the game until all reds have been cleared. Once all red balls are off the table, the coloured balls are no longer replaced and must be pocketed in ascending order of their point values to conclude the frame.12