Two interfaces, one system
For most everyday tasks, the Settings app alone is enough, but a handful of older features have never been rebuilt for it and still live only in Control Panel. If you can't find something in Settings, checking Control Panel is usually the fastest way to track it down.
Some legacy settings haven't moved yet
Internet Options is a good example of a feature that predates the Settings app and still governs security zones and trusted-site lists that most modern settings screens don't expose at all. Advanced network adapter settings and certain legacy printer options work the same way -- still only reachable through the older interface.
Frequently Asked Questions
Will Control Panel disappear from Windows eventually?
Microsoft has been moving features to Settings gradually for years, but not everything has migrated yet, so Control Panel is likely to stick around for the foreseeable future.
If the same setting exists in both places, does changing it in one affect the other?
Yes. Any setting that exists in both interfaces controls the same underlying system value, so changing it in either one applies identically no matter which interface you used.