Showing how other are using your product proves it's added value.
When you show reviews, ratings, testimonials or other forms of feedback from other customers, you give potential buyers proof that the product is being used and valued. It reassures that they're not alone and making the right decision.
Recently I've seen some (often absolute) statements going around, generally in the line of "open source commerce platforms are a terrible idea". Now of course different solutions always have different pros and cons.
A hierarchy of evidence (or levels of evidence) is a heuristic used to rank the relative strength of results obtained from scientific research. I've created a version of this chart/pyramid applied to CRO which you can see below. It contains the options we have as optimizers and tools and methods we often use to gather data.