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It’s worthwhile to be explicit there. For example, a person can be a heavy user of Pandas library but rarely use underlying numpy library directly. There could be motivation to contribute to numpy with expectation that Pandas will benefit too.

Based on the article, this user should start learning to use numpy directly, and I can agree with that.

why I would want to contribute to a project I don't use

Sometimes I might have an academic or research interest in a field I'm not actually working in or I read about a topic that sounds interesting. Then contributing to a project working with that topic or field might seem like a good way to learn and get more into that topic.

The simple answer would be you don't have to. The underlying assumption is someone already have personal or professional interest in those projects.