I have limits on all my axes, using mechanical ones without levers. I find the levers bend over time and can give slight variances to ones' home position and the work start position. This can be a problem, if doing repetitive cutting after etc.
Depending on how one sets up the material in the cam programme and how you zero the Z axis on the table, can in the beginning, can result in having the Z dive into the spoilboard, if not paying attention to what one is doing.
Putting on the limit for the Z axis can help in stopping it dive right through the spoilboard.
I am using leadshines' mx 4660 with Mach3 and all limits wired in series.
Most people seem to zero the Z to the top of their material. Easiest, especially when not cutting the full depth of the material.
I started to zero to the spoilboard and mic the material each time before cutting to ensure that I do not cut into the spoilboard, adjusting the cam toolpath if needed. This way I have set the limit of the Z to 0.1/0.2 mm below the surface of the spoilboard.