Question

why are straws used to drink soft drinks

Answer 1

mostly drinks are acidic in nature
so
if they have direct contact with teeths , may be harmful
in order to avoid this
straws are used

Answer 2

Constant, prolonged exposure to sweets and drinks may cause cavities and tooth decay. straws avoid this tooth decay as less time will spend in contact with teeth.  that is y it is used to drink soft drinks.