Question

why do we use EBT for determination of hardness of water

Answer 1

When used as an indicator in an EDTAtitration, the characteristic blue end-point is reached when sufficient EDTA is added and the metal ions bound to the indicator are chelated by EDTA, leaving the free indicator molecule.

Answer 2

specific types of indicators are used for specific reactions
because the pH of different solutions are different so the indicator we will choose the indicator whose pH is same as your solution.