Connect analog load cell to usb via AVR
Are there any benefits of using such combined chips?


Yes. They are simpler and allow you to get away with less engineering work. Think 'Plug and play'


  And if there are, how the digital output of such chips should be read with the AVR controller?


It depends on the device. This will be explained in its datasheet. Most likely it will have a SPI or I2C interface.