deactivating the pull-up resistor

hi,
sir i want to know that why us deactivate the pull up resistor in freeimu library without deactivated i can still read I2C data .thanks

code:

  // disable internal pullups of the ATMEGA which Wire enable by default
  #if defined(__AVR_ATmega168__) || defined(__AVR_ATmega8__) || defined(__AVR_ATmega328P__)
    // deactivate internal pull-ups for twi
    // as per note from atmega8 manual pg167
    cbi(PORTC, 4);
    cbi(PORTC, 5);
  #else
    // deactivate internal pull-ups for twi
    // as per note from atmega128 manual pg204
    cbi(PORTD, 0);
    cbi(PORTD, 1);
  #endif