“Send” just sends the file to the laser. “Start” sends it as a temporary (unnamed) file, and the laser begins running the job when “enough” data has arrived. (I’m not sure what counts as “enough” in this case - it’s up to the hardware).
If you are sending a job that has photo engraving, or high-speed engraving, those can take a decent amount of CPU horsepower to execute the job, and receiving the file also requires CPU time. If the job is complex enough, it’s possible for the controller to simply not have enough compute power to do both tasks, which can result in dropped packets, or the image engraving slipping. Using “Send” means the controller is only doing one thing at a time, and is less likely to mess either of them up.
If you hold the Shift key when you click Send, LightBurn will auto-start the job after a successful transfer, saving you having to find the file on the machine and start it through the menu.