Cannons in urf enhance the snowbally nature of the gamemode
In two ways. One, the losing team’s mistake: the cannons make it so that the winning team gets kills delivered to them more regularly, and the self cc at landing leads to an easy death and kill for the winning team.
Two: losing teams rely more on splitting and backdooring, neither of which are possible when the winning team has the easy to use cannon to have a nigh perfect defense.
Bonus round: the winning team can use the cannons to easily steal or sneak elder dragon, much moreso than the losing team can (as they kill the dragon and also the losing team faster).