In the image below a small cargo box was jettisoned and one slot is empty now in the ship that can be used to add another cargo box
Why would you try to jettison your cargo?, well in some cases if pirates are behind your ship you may be able to accelerate faster if you drop some of your cargo and live the day, some pirates may even try to get your cargo instead of following you, or pirates may ask you to jettison your cargo or die. Sometimes you may reach some systems with illegal cargo and if the space police or federation finds you with the illegal cargo in your ship you may face big fines or get into real trouble or be killed, so by jettisoning your cargo and destroying it before they catch you can save your life.
Illegal cargo? there will be some illegal items depending on the system you are trading with, like illegal weapons, drugs, slaves, etc... etc... some crew members may not feel confortable with that, so be careful on what you load in your ship!
And if look carefully in the image, there is a SOS button too!, so if the player gets in distress for example no food, oxygen, fuel or even barely alive with lots of ship damage, you can ask for help and if lucky survive the day, but it may cost a lot of money to buy fuel like this.