I would agree with Pascal's reply.
The area of materials is expanding so it is constantly changing and having an understanding of their properties is beneficial especially when it comes to material selection for design.
Understanding manufacturing processes is important to know that what you are designing is being designed in a way that can be manufacturable in a practical manner. On the job, you'll likely be collaborating with manufacturing engineers to be able to optimize product designs for this reason.
Product appearance combined with functionality to meet customer needs/specification will be a primary product requirement particularly so that you can meet product verification and validation requirements.
Additionally, in my experience, particularly in any product design capacity, especially for medical devices, one of the first things that will be required is performing a risk analysis from a design perspective but also from a manufacturing and also usability point of view. Typically, a Design Failure Modes and Effects Analysis (DFMEA), Process Failure Modes and Effects Analysis (PFMEA) and a Usability or Application Failure Mode and Effects Analysis (AFMEA) will need to be performed to understand how a product can fail and evaluate the severity of failure, frequency, probability of that failure occurring but also determining what steps need to be taken to mitigate those risks.
I hope this is helpful. If not, feel free to comment that more information is desired.
2- Designing stuff that can actually be manufactured economically. This is something you learn "on the job" by making mistakes. It is good to work closely with people in manufacturing in order to speed up your learning.
3- Industrial design: making stuff that looks good