Why is there a guard band in ADSL
The guard bands are required because filters aren't perfect.

A filter that can pass 8,000 Hz but block 8,001 Hz must have a group delay of at least 1s. Building a circuit that a signal takes 1s to pass through requires an awful lot of material and space, so we have to make do with a flatter slope, e.g. passing 8 kHz but blocking 10 kHz, so any DSL traffic can only use frequencies that are adequately blocked in the filter for the analog line.

The same thing is between up- and downstream. The receiver uses a filter to block out the signal from the transmitter, allowing a higher gain on the receive amplifier.