Why specifically use Passata, rather than tomatoes, in Ragu?
The primary reason is definitely convenience. If you don't want skin and seeds in your sauce, then you have to do some work to avoid it. Yes, it's possible, e.g. passing through a food mill, or blanching and peeling plus retaining only the flesh, but having it already done is a whole lot easier.

Passata is usually much thicker too, so it won't need as long a cooking time to reduce into a non-watery sauce as fresh tomatoes. On top of that, you don't have to worry about whether you can find good fresh tomatoes, e.g. if it's winter and they're out of season.

Your two cases for comparison seem to overlook this: if you just chop and cook, you'll have skin and seeds in your sauce, as well as excess liquid to cook down, while if you use passata, you won't. (Also, for what it's worth, you'd need pretty giant tomatoes to get the equivalent of 3/4 cup of passata out of one tomato.)

Of course, if you don't mind skin and seeds, it's also perfectly fine to simply cook fresh tomatoes and be done with it.

Note that you can often get similar advantages from other canned tomato products. In the US, crushed tomatoes are far more common, and a roughly similar texture. (Peeled whole tomatoes, diced tomatoes, tomato sauce, and tomato paste can also be useful depending on what texture you're aiming for.)