Its all about security updates in your case. There is many severe security flaws being fixed, especially of you are on 15.0 today.
The few I know that are running older devices that can't upgrade further are running this version and it seem to run fine.
Most of them have the iPad air 2 or iPhone 7
There are generally comparisons in terms of performance and battery time on the web. Generally the first release of a major update also contains bugs that are fixed in later releases.
You can read more about what all the updates has improved/changed here.
iOS 15 brings audio and video enhancements to FaceTime, including spatial audio and Portrait mode. Shared with You resurfaces the articles, photos, and other shared content from your Messages conversations in the corresponding app. Focus helps you reduce distractions by filtering out...
support.apple.com
Not all changes and bugfixes are listed, but it still gives you an idea of what the major changes are.
Almost all of the updates has security updates, they are listed in a separate link.
Not all are severe but there are some that has a high CVE/CVSS score.