As I'm going to use a Wasatch spectrometer in my lab, I'm ready to refer the manual. However, I did not find very detailed information, so I go to Thorlabs.

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_ID=5276

It mentions as below,

The vertical axis of the spectrum can be displayed as Absolute Power or Power Density, both of which can be displayed in either a linear or logarithmic scale. In Absolute Power mode, the total power displayed is based on the actual instrument resolution for that specific wavelength; this setting is recommended to be used only with narrow spectrum input light. For broadband devices, it is recommended that the Power Density mode is used.

So why narrowband for absolute power while broadband for power density?