The 8051 data bus is multiplexed with the lower 8 bits of the address bus. The 573 is used to seperate these two busses from each other to get a full 16 bit address bus and an 8 bit data bus again.
In fact the 573 freezes the contents presented on it's inputs when the ALE pulse arrives, making the lower 8 bits of the address available on it's outputs. After that the 8051 switches to data bus mode to receive or send the data.

Happy programming.