Carrousel7956 What can't you do in desktop mode that you can do on a laptop.
This question doesn't have a concrete answer because it depends on what you need to do and what tools you use.
To make a naive example, Signal on your laptop will be able to present your screen to others in a video call, while this is not be possible using Signal on Android in desktop mode.
This example underlines how the same tool might behave differently depending on the OS (different possibilities -> different implementatins), forcing you to search for a workaround or another tool to achieve the same.
Also, on GOS you lack root permissions, and this will prevent you from performing some actions, like a SYN scan with nmap, or opening a port in the known range (1-1024), or making use of a local server (like a webserver for testing purposes or ssh server).
Sometimes the limit is not even the Desktop Mode itself, sometimes is the architecture of the device (ARM vs x86).
Some other times the limit are the developers not supporting the Desktop Mode (like not mapping a keyboard with shortcuts, and so on).
All in all it depends on what your needs are, for the average user Desktop mode might be sufficient.
There are ways to achieve almost everything also on GOS, but sometimes this comes with so much effort/workarounds that might kill the purpose of having stuff done on your smartphone in Desktop Mode.
I am a big fan of Desktop Mode myself, but I would choose a different tool depending on what I have to do.
I think you would agree that you wouldn't operate a person with a chainsaw, nor cut a tree with a scalpel :)