CBSE Class 10 Answered

When the ureter enters the bladder it travels through the wall for a distance creating a tunnel so that a flap valve is created. This valve prevents urine that is in the bladder from backing up and returning into the ureter. Thus, when the bladder fills and later when it squeezes down to empty, back-up (that is, reflux of urine) is prevented because of this valve.

This valve also lets you get rid of the urine in the bladder in one go, as the urine cannot travel anywhere else.