Normally there are times when users have trouble reading on their phone when the text is too big.
The solution to that has always been to change the font size overall but it is to be noted that there are some apps on your phone that already have large fonts and the settings you just applied may not work for every app.
This mostly leads to users going for the conventional method that is buying reading glasses.
But with iOS 15 on the iPhone, users can now adjust the size text for each of their respective applications.
In other words, if let’s say Gmail has a small font size you can increase it, and while for other apps which have large font you can adjust the size only for those applications.