There's a lot of design systems.
Being in an environment where new projects are coming in every other day, I had to constantly make important design decisions even when I thought the designs were not yet 100%. Pivoting between many different brands allowed me to notice patterns that emerged across different brands and how similar they actually are. It also reinforced my understanding of why certain colors are used in different industries. For example, the color 'red' is used in the food industry because it evokes a feeling of hunger.
In addition, each company had their own style and brand guidelines whether it be the colors, fonts, or company values. It was essential to understand and implement these guidelines accordingly. When someone utters the word 'swoosh' what brand do we immediately think of? That's why brand identity is important.
TLDR always be on-brand all the time.