74LS245, like most bus-oriented chips, has tri-state drivers. In the third, high-impedance state, the driver is effectively disconnected, allowing other drivers to use the bus. If you just connected the busses, you could have only one data source.
Circa Tue, 08 May 2007 07:34:01 GMT recorded as looks like Rikard Bosnjakovic sounds like:
When there exist busses C, D and so forth. This device is meant to be controlled in such a way to permit communication between internal busses and a common data and/or address bus. So call the data bus bus D. Bus A needs to read data from bus D. Enable the 245 to permit bus A to receive data from bus D, while disabling the transceivers on the other busses. Likewise for B, C etc. Reverse the direction when bus A has data to place on bus D. Rinse, repeat.
Another example would be data routing. Enable bus A to transmit to bus D, and bus B to receive from bus D. Data now is connected to pass from A to B. Shut off bus B and turn on bus C to receive and.... Get the picture?
Think of it as an amplifier. Say you have a CPU. You can connect the databus directly to some memory, but once you start adding a lot of circuitry, it would load down the CPU's data bus too much (the CPU datalines couldn't supply enough current). So you add a buffer so there won't be much load on the CPU, but there will be enough signal on the databus for all the peripheral circuitry.
It's a transceiver because the databus is going both ways. You could use two sets of one-way buffers, to buffer signals out of the CPU and to buffer signals going into the CPU, but that would be bulky. There is enough use for this that it was worth creating a single device for it.
And given that many of the answers here have addressed the tri-state nature of the device, you can also turn off the transceiver, so the CPU databus is isolated from the rest of the circuitry. Not so useful in itself, but if you had two CPUs using the same rest of the circuitry, disabling the databus transceiver connected to one (as well as the buffers on the other CPU's lines) would allow the other CPU to take over. WIthout the bus transceiver, they would fight over the bus.