The technical name for this is "progressive web app". Basically, it's a website built a certain way with some additional resources that allow it to appear to be a regular full-screen app and run independently from Safari, I'm developing one myself. From the developer's standpoint, it's much easier because you don't need to register anything with Apple, don't need to pay any fees and don't need to get your app approved. When the developer updates their app it's not necessary for the user to do anything, the new code is automatically loaded when the app opens (although caching can be a problem). So the developer can basically do whatever they like, which might be a disadvantage from a user standpoint.
However, there are some other factors that should make web apps more secure by nature. The biggest one is that web apps cannot access the device filesystem like native apps. They are completely "walled off" and if the app needs to store any data on the device, it is done with some special API's that store data in designated places instead of simply reading and writing to files. If the web app needs to export data, it can only write to the download directory instead of saving a file anywhere. If it needs to load user data, that can only be done with a special API where the user manually chooses the file. A web app cannot programmatically open a user file because it cannot "see" the filesystem.
There are also limits to the amount of data that a web app can store on the device. I have found it confusing to understand exactly what that is, but depending on the techniques that are used it appears to be less than 1gb. This is different on iOS, Android, Windows and MacOS which makes things even more confusing for developers.
Another limitation of web apps is that they can't run in the background. I'm working on a mapping/gps app and if the user leaves the web app to answer a call or read an e-mail, the web app will stop tracking their position. A native app could continue working in the background in that scenario.