Prescriptions aren't only for addressing medical abuse of patients — they're also for addressing the medical abuse of practitioners.

Imagine if I were some Dr. Oz quack. If I really wanted to, I could make a PTSD app right now and release it on the app market. Doesn't matter if it actually works. I'd have to avoid representing myself or my product as a legally qualified medical practitioner, but I could use one of the many non-legally protected "medical" titles and trick people into thinking I'm a doctor and purchasing my app.

An prescription app with FDA approval is significantly different from the quackery above. FDA approval/prescription status gives an app legitimacy. In order to get FDA approval, the app needs to undergo rigorous testing and analysis to prove that it is safe and effective when used as instructed for its intended use. Having it locked behind prescriptions also lends it legitimacy, because it means that only licensed medical professionals are allowed to distribute it. Another point is that doctors can better instruct patients on effective usage of the app as well as adjust patient medical care with this app in mind, as prescription-level things tend to come with a lot of accompanying medical literature useful for understanding them.

Those are the two good reasons: apps being granted prescription-status/FDA approval separates them from BS apps, and having the apps prescribed by doctors allows doctors to instruct patients and tweak it to better fit patient care.

Of course, there's also the third bad reason... capitalism. Getting anything FDA approval takes a ton of time and money, and at bare minimum companies need to recoup the cost. Beyond that, companies also wanna make a shit ton of money and bleed the populace dry. The market for prescription apps is in its growing stage right now, there's little to no competition, and no government-mandated price regulation. These are the perfect conditions for companies to form monopolies and mark up their prescription apps exorbitantly, forcing people to buy their products because no alternatives exist.