Interesting question, I'm not sure what is the purpose of this question but I'll answer it as best as possible :)
I have been contributing to Clarity since it started pretty much, at some point even helped with some feedback on some possible iterations and updates a few years ago. I love what Clarity offers which the way I see it is not about the calls or the question and answer but the opportunity for experts and those with questions to bridge the journey between that problem awareness to getting their problem solved - either by calling someone, getting an in-depth response written to them, or hiring them or their companies directly based on the knowledge provided through this platform. As a marketer, I see the journey as Awareness, Consideration, Decision - Clarity helps someone with a problem bridge that realization right onto the decision of fixing something.
Quora, on the other hand, does offer some cool features such as requesting certain people to answer but their positioning is not targeting to build a community of 'experts' so the answers you get might not be worth the wait. Quora doesn't encourage you to schedule calls instead it monetizes off ads and partners.
Clarity is meant specifically for businesses and startups - not random 'everything questions.'
Quora is a place to gain and share knowledge. It's a platform to ask questions and connect with people who contribute unique insights and quality answers.
Make faster & better decisions to grow your business. Clarity is your lifeline that instantly connects you with battle-tested advice from entrepreneurs.
One of the biggest of why I just love Clarity is because its part of a bigger community of entrepreneurs and business owners and experts who reach out to each other and are 'there' for each other whereas with clarity - is too generic and there is no community in place.