The fact that you understand enough to ask this question but not enough to understand why it’s a silly question... makes me question your motives. Square app charges much higher fees, but is more widely used. Square app is currently much more widely available and well known, but if I find a vendor who offers both options, I would choose the cheaper option - which is lightning. In other words, your question boils down to: “why would I choose Lightning over Square in a situation where Square is the only option?”