my current project used d4 &d5 as bluetooth, but i want to make it more simple using display as output. but many project using a4 & a5 pins for i2c 16x2 display.
That would be because A4 and A5 are the pins reserved for the hardware I2C interface. 
So in fact, all projects using I2C would be using A4 and A5 for that. Including any Bluetooth device using I2C since I2C is a "bus" and the same pins are used for all devices at once.
(Unless anyone wishes to indulge in "creative headbanging" and making abstruse objections to these generalities. 
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