But as mentioned, the audience is captive. If there was another parking option around the corner they would have used that. I don't think parking lots care how long it takes you to download their app and pay.
A parking lot clip could use the time-based notification to warn you that your meter is almost up and give you the option to re-up. Without having to walk back to the lot to do it. I imagine this would be especially useful in a tourist-heavy area since somebody who isn't sticking around won't want to download a whole app for the purpose.
Any real world good or service would fall onto Apple Pay, which makes its money through a micro fee on the card issuer, basically to use the phone as a convenient, highly fraud-resistant card.
App vendors are only paid by the people who use the app, so they have every incentive to make it that much easier for some one to use the app than the ticketing machine.
The parking lots may be competing with other lots, but really they are most likely getting money either way, they want to decrease their overhead, hence apps.
The reality is that most parking lots do need to have fallbacks for these cases rather than telling people to drive off, and those fallbacks are often more expensive in terms of staffing/maintenance (which is why they added the app in the first place). Something where I point at a code on a sign, it pops up a payment button without me knowingly downloading anything will increase the cost savings beyond an app.