The medium is the message, and the message is “You’re too cool to do it the low-tech way.”
Using a smartphone to rent a car is superfluous at best, annoying at worst, but its utility is beside the point. Given the personalized drop-off service, this is technology for show, produced for rhetorical effect rather than functionality. The point is to have an app, to interact with customers by text rather than voice, to be with-it and modern. When tech startups flaunt their apps, they’re often pandering to an audience that identifies with mobile and web technology, rather than one that needs to make use of it.