From the interviewee side: It gives you a known set of material to prepare for (ofc there may be newer questions). But this alone helps a lot. With the explosion of frameworks, libraries, languages it is very hard to prepare for everything.
From the interviewer side: Again it will give you a standardized set of questions which can be asked and you can compare across candidate due to this.
>> about half of them admitted they already use my open-source software
Similar to how people don't have time to prepare for leetcode, there may also be people who don't have github repos with open source code etc. Take my case, I work for a company where I have to get permissions if i am gonna start a open source project, even though it may not be related at all to my current work.