I don't think $A000 is much more common than any other memory address. 8K cartridges will be located to $8000, and for other software you can store them pretty much anywhere in memory.
However, the VIC-II chip can read/write to 16K at a time. The default setting is the first block $0000-$3FFF, but many programs rather relocate it to $C000-$FFFF because there is RAM underlying ROM, and more difficult to use for other purposes. The VIC-II chip can't access the ROMs anyway, so for that purpose it is ideal to store graphics data in that section.
As to Mike's question: No, there is no built-in monitor in the C64. Likewise there is no single-step flag. However, using a cartridge like The Final Cartridge (III) or Action Replay (V or VI), these contain monitor programs that let you set breakpoints. These are intercepted and from there single-step instruction by instruction once inside the monitor.