Hi there!
It is necessary to rinse the inside of an Erlenmeyer flask with deionized water periodically to wash the flask without adding ions which could affect the titration.
If you wash an Erlenmeyer flask with regular tap water, you are adding ions, like sodium or chlorine, which could make you miss the equivalence point by only tenths of mL (that's a big difference when titrating). This is why when washing you use deionized water to clean the flask while ensuring that your titration is accurate.
Have a good day!