You're of course correct. (As are the many other replies that amount to: "Just don't use the phone while driving, dummy!")
However, it's worth keeping in mind how the human mind works; in particular its limitations. Our minds and behaviours are inherently flawed. Part of being a smart and responsible person is not just modulating your behaviour, but also designing your life so as to elicit the right kinds of outcomes. A simple example is putting an item that you want to bring with you tomorrow by the door. You could "Just remember to grab it when you leave tomorrow morning!", but you're accounting for your own fallible memory by putting it by the door while you're thinking of it. Another example would be a person who puts a tempting snack on an inaccessible shelf: they buy the snack because they want to have a treat sometimes, but they purposefully make it slightly inconvenient for themselves to eat the snack, so that they don't just reflexively eat it all the time. It's part of a strategy to invoke more rational thinking, rather than just let your immediate impulses win.
There are many more examples of such behaviour. Obviously it's "better" to simply have infinite willpower and rationality; but for people who do not (and if we're being honest, this describes all of us; though our individual temptations and biases are different), it can be useful to design your life to account for your fallibility.
So, in principle a cellphone app that disables the phone while driving can be useful. It's for people who recognize that it's a really bad idea to use your phone while driving, and yet are so addicted to their phone that they cannot avoid answering it when it rings. (Or are so addicted to status updates that they will absentmindedly check when bored, even if they are driving!) These people may also not have the discipline (or memory) to (for instance) always put the phone in the trunk before getting behind the wheel. For those people, such an app can be useful.
Having said all that, I think it's unrealistic to expect an app to properly differentiate between the situations where you would want the phone disabled (while driving) and those where you don't (parked, passenger in a car, etc.). So I think the question-poster should instead investigate other ways to modulate their own behaviour (e.g. put a holder in the car, in a very visible location, that says "PHONE BATTERY GOES HERE", and always pull out the battery before turning on the car).