In many UX jobs, you will be the one who has to create a design system, generally dictated by marketing and branding guidelines.

I recommend UX designers design posters because posters are the ultimate test of layout. Especially band posters, where you have to lay out a huge amount of information clearly and beautifully, which builds skills with respect to element hierarchy, relational grids, and eye flow, all of which are vital when laying out apps or websites.