what is benifit to update minimum app target to 10 or 11 or 12 or 13 or 14 or 15

The benefit is for you (the developer) not having to support all those old versions, and to be able to use new and improved APIs introduced in newer versions.

Do you actually have test devices running all those old versions? That’s a lot to manage. The simulator is great too, but the current Xcode supports simulator versions only back to iOS 12.4. If a user reports a bug on affecting only an older version on a specific device model that you don’t have and don’t have a simulator for, what do you do? And if you release an update, how broadly can you test it?

If you want to use newer functionality, do you clutter your code with version-specific conditional logic to make it work? That can get tedious and error-prone. And in cases where newer APIs improve on older APIs (such as, for example, the fantastic diffable data sources for tables and collection views, added in iOS 13) then being unable to use them can be deeply frustrating.

And in the Apple world, users adopt newer iOS versions much faster than Android, if you’re familiar with that world. So you’re likely to find the proportion of users on iOS versions 9, 10, 11, 12, and even 13 to be very small (see charts here) and arguably not worth the effort. Our poor Android counterparts have a much tougher time supporting all the OS versions still in wide use on devices of similar vintage.

A good practice is to keep at least 3 older versions possible: 12, 13, 14, in addition to 15. Doing so you are sure more than 95% of users will be able to use your app.

Another benefit for you is to be able to develop on older Xcode versions that do not support 15.

In this specific case, I would argue there's no point to keeping iOS 13 and 14 around, as any iPhone that can support those OSs can support iOS 15.