Question

why does atmospheric pressure help in drinking juice with straw​

Answer 1

When you suck on the straw you are lowering the atmospheric pressure inside the straw. The pressure of the air on the top surface of the drink then is pushing down on the liquid inside the cup with more force than the pressure of the air inside the straw pushing down on the surface of the liquid in the straw.

Well, think about what happens when you put a straw into liquid. ... When you inhale through the straw, you take the atmosphere out of the straw (it goes into your lungs). Now the atmosphere is pushing down on the liquid in the glass, with noting the balance it inside the straw, so the liquid rises…into your mouth!