Cargo space is for more than just trading - its a critical part of managing your ship modules at any moment.
You are never really meant to have everything you could possibly need on your ship all at once - you should expect to "uninstall" modules you are not using and swap in ones that provide the necessary bonus you are looking for. This is where cargo space is critical - you will need that extra storage space for modules that you deactivate. Thus, a healthy balance of both is required for optimal flexibility.
Hope that helped