Why data and address buses are tristated in 8085?

In the general case, a bus is tristated to allow more than one source to drive it. This can be the simple case of a bi-directional bus, or the more complex case of a multi-master bus configuration.

In the 8085, the address bus is tristated when a HOLD/HLDA sequence is in process, allowing another bus master to control the bus. The data bus is tristated for the same reason, and also to allow bi-directional data flow (read vs write) to occur.