2 cents from my side.
I'm considering you're talking about using widely known libraries.
Extension: It gives you an idea on how it works, now it's quite easier for you to extend the library
Learning: It is written by experts, you'll get a lot to learn on how would you write the code (Design Patterns, Architectural Style, Optimisation)
Interviews: Believe me, when I share internals of how Java works or how Spring works, it gives an awesome impact on interviewer
Contribution: Rare scenario, but if you find any leak or bug you'll can sharing your inputs. Voilla!! You're an Open Source contributor.
Code Review: Congratulations!! You're awesome code reviewer, you know lot many standards as you've gone through em.
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