I am guilty. When I have to interview somebody I will check out their github activity, and other development activity (forum/blog posts, etc.).
If there is enough activity it will show more what kind of developer they are than their resume will. Especially when they have personal projects.
It will also give me discussion topics both sides are probably interested in. I can see the things they made, and can discuss why they made certain decisions. Get into their thought process.
Generally the work you do at a company is something you have to do, not something you want to do yourself. Work in your github is quite often something you did want to do yourself.
And while I'm checking out their activity I might also find interesting/cool software/techniques/etc. I did not know about.
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