I was having a ponder and it struck me that the amount of Z travel quoted on a machine makes little sense. Or I've lost the plot and would like to be enlightened

Say you have a Z travel of 200mm and wanted to machine pockets 150mm deep in a 200mm piece. Your functional end depth will be dictated by your end-mill length surely! As soon as you run out of end-mill, the bottom of your spindle will hit the top of the workpiece.

So you have gantry height (-Z axis mount etc.) dictating the height of the workpiece.
Then max. end-mill length and Z axis movement dictates maximum cutting depth.

Of course you can also flip the piece for double the effective cutting height if the design allows it e.g. a through pocket could be machined from both sides.

Does this make sense? In what scenario would a Z travel longer than a useable end-mill length be helpful?