Does Z travel make sense as a number?

[routercnc]

Similar to Clive’s response you might want to drill holes in the end or side of say a rectangular piece of material so need to have space under the tool.
As mentioned being able to move the spindle up and down in the holder is also really helpful and extends the range of the machine.

If you are making a router then 150mm from the tip of the tool to the bed is a good start point. This is because it allows a modest vice and work piece to fit under the tool whilst not requiring the Z axis to have to hang down too far from the gantry when machining parts down on the bed. Long travel on Z will reduce stiffness.

If you are making a CNC mill then you can make the column taller to give more options.

Of course this all depends on what you need to make but a win in one area will comprise another area.

Also being able to retract above the work and get your hands in for manual tool changes is also helpful.