Does Z travel make sense as a number?

[Voicecoil]

There is sometimes an advantage to being able to move the spindle up and down on the Z carriage a bit to accommodate different cutter lengths provides you have clearance.

[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.

[JAZZCNC]

Many reasons for a high Z-axis but no right or wrong for either, it's all down to usage.
For instance, I recently built a machine with a lifting gantry that had 400mm of Z-axis travel but was never going to cut deeper than 2 or 3mm, the height was needed because of the part size.

I've built similar machines with high gantry's and Z-axis because they wanted to drill holes thru 10" beams and machine scarf joints, pockets, etc for log cabins, yet still, they wanted to engrave slate which was only 5mm thick.!
In cases like this, you just have to accept that there is a price to pay for a large Z-axis. Raising the bed is an option for thin material but it soon gets tiresome having to surface the bed every time you want to machine something thin.

Also, regards pocketing 150mm deep then you better have deep pockets, have you seen the price of tools over 100mm in length.?

Drills are probably the longest thing you'll stick in a spindle and a good reason for a decent amount of Z-axis travel/clearance.